Sitthixay Ditthavong / AP

A large group of public school teachers marches past John Marshall Metropolitan High School Wednesday on Chicago's West Side. Teachers walked off the job Monday for the first time in 25 years over issues that include pay raises, classroom conditions, job security and teacher evaluations.

Updated at 3:31 p.m.. ET: CHICAGO -- Classes for the nation’s third-largest school district were canceled for Friday as the Chicago teachers set out to strike for a fifth day, according to NBCChicago.com.

 Negotiators trying to bring an end to the Chicago teachers' strike had said they were confident an agreement would be reached soon, but union leaders cautioned parents it was "highly unlikely" students would return to school Friday.

Chicago Teachers Union president Karen Lewis said she hoped an agreement can be reached by the end of Thursday, the fourth day of the strike.

Lewis told reporters she doubts teachers would be back in classrooms Friday, but said she's hoping for a Monday return. "Oh, I'm praying, praying, praying. I'm on my knees for that, please," Lewis told NBC Chicago. "Yes, I'm hoping for Monday. That would be good for us."

Chicago Public Schools chief education adviser Barbara Byrd-Bennett was even more optimistic, saying she was trying to get students back in class by Friday.

"The conversation was productive," Byrd-Bennett said on Thursday. "There was steady and substantial movement on key issues around teacher evaluation and layoffs and recall.”

Chicago's teachers in the nation's third-largest school district went on strike Monday for the first time in 25 years in dispute of education reforms sought by Mayor Rahm Emanuel.

For the first time in days, Emanuel's chief negotiator, School Board President David Vitale, agreed with Lewis' summary of the talks. Only 24 hours earlier, Vitale had threatened not to come back to the negotiating table until the union put forward a better offer. 

"We had a very productive evening," Vitale said. "We all go away hopeful that we can go come together on this." 

350,000 kids out of school
With more than 350,000 children out of school, the patience of parents had begun to fray as hopes of a quick resolution to the biggest U.S. labor strike in a year faded. 

Earlier in the day, civil rights leader Jesse Jackson, who is based in Chicago, appeared at the site where negotiations were supposed to take place on Wednesday and said that he had met with both sides separately to urge them to settle. 

Lewis said the progress on Wednesday was on the two most vexing issues -- using student test scores to evaluate teachers and giving more authority to principals to hire teachers. 

 Chicago mayor: Get kids in class during contract talks with teachers

"We made significant progress on the teacher evaluation side of the equation," Byrd-Bennett told NBC Chicago. "Clearly we're remaining consistent with not wanting to lower the standards for our children. ... I think there were really good discussions."

 But Lewis said Thursday, there's still much work to be done.

"We haven't even talked about the professional development side," she said. "We want to make sure this is done right. Doing something fast is not the way to go. Haste makes waste."

The union is concerned that more than a quarter of its membership could be fired because the teachers work in poor neighborhoods where students perform poorly on standardized tests, which Emanuel wants to use to evaluate teachers. 

Lewis also said the union fears Emanuel plans to close scores of schools, putting unionized teachers out of work. 

Lewis led the walkout on Monday of more than 29,000 teachers and support staff, saying the union would not agree to school reforms it considers misguided and disrespectful.

Question at heart of Chicago strike: How do you measure teacher performance?

The dispute jolted the United States, where a weakened labor movement seldom stages strikes and even less frequently wins them. Organized labor has lost several fights in the last year including Wisconsin stripping public sector unions of most of their bargaining power, Indiana making union dues voluntary and two California cities voting to pare pensions for union workers. 

The strike in Barack Obama's home city has also put the U.S. president in a tough spot between his ally and former top White House aide Emanuel and labor unions Obama is counting on to win re-election on November 6. 

Obama has said nothing in public about the dispute, allowing administration surrogates to urge the two sides to settle. 

Obama's own Education Department has championed some of the reforms Emanuel is seeking, and a win for the ambitious Chicago mayor would add momentum to the national school reform movement. 

'Difficult for us to understand'
The city is operating 147 schools with non-union staff to offer meals and "keep children safe and engaged," but only a fraction of parents have been using that option, officials said. 

At Disney elementary school, several dozen strikers with homemade signs targeting Emanuel and school policies picketed in cool, sunny weather on Wednesday. 

 Union leader to Chicago teachers rally: In for the long haul

Kent Barnhart, a music teacher for the past 25 years, said neighborhood parents had been supportive, offering water and opening their homes and even joining picket lines to march. But he said teachers were frustrated with the slow talks. 

"It's difficult for us to understand why they have not truly discussed over the last 11 months things that have been very important," he said of school officials. "It didn't seem like they took it seriously -- really important things like evaluations, health benefits and pay." 

Both sides agree Chicago schools need fixing. Chicago students consistently perform poorly on standardized math and reading tests. About 60 percent of high school students graduate, compared with 75 percent nationwide and more than 90 percent in some affluent Chicago suburban schools. 

The fight does not appear to center on wages, with the school district offering an average 16 percent rise over four years and some benefit improvements. 

More than 80 percent of Chicago public school students qualify for free lunches at school because they come from low-income households. 

NBCChicago.com's Michelle Relerford and Lisa Balde contributed to this report, as did  The Associated Press and Reuters.

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Chicago Mayor Rahm Emanuel is ready to "finish it" already.

As Chicago teachers picketed for the third day Wednesday, Emanuel said there's no reason students can't return to classrooms while union representatives and school officials hammer out the final contract issues.

"There is nothing that can't be worked through while our kids stay in the classroom," Emanuel told reporters in a news conference ahead of Wednesday's City Council meeting. "My staff, as well as the Chicago Public School leadership team, is committed to working through these issues, never leaving the table to get this job done. And those issues can be negotiated simultaneously while our kids are in the classroom learning."

Negotiations continued as teachers attended three rallies and continued to picket outside the board of education headquarters. The union said it submitted a new written response to CPS' latest proposal, handed down Tuesday night.

Scott Olson / Getty Images file

Mayor Rahm Emanuel visits Chicago Public School children at Woodson Regional Library, Sept. 10, 2012.

"We've looked at their comprehensive package," CTU president Karen Lewis said before heading into negotiations. "There's some movement forward, there's some movement backward. What we'd like to do is continue to move forward."

For more visit NBCChicago.com.

CTU vice president Jesse Sharkey said his biggest concern with the proposal was the tone.

"[It] seemed to say, 'This is it, we're drawing the line in the sand, now the union's gonna have to come to us, we've done all the compromising we're going to do,' and I think that's a really harmful tone to take in a negotiation," Sharkey said.

It's not clear how much longer negotiations could take.

Earlier, Sharkey told reporters the school board's latest proposal isn't much different than the previous proposal and mostly just moved things around. 

Emanuel and school officials said only a couple key issues remain to be settled, but union representatives said there are 43 left to tackle.

Question at heart of Chicago strike: How do you measure teacher performance?

Sitthixay Ditthavong / AP

Public school teachers cheer as Chicago Teachers Union President Karen Lewis, unseen, arrives unexpectedly to address a rally of thousands of teachers outside the Chicago Board of Education district headquarters on Tuesday, Sept. 11, 2012.

When asked what it hopes to accomplish Wednesday, Chicago Board of Education vice president Jesse Ruiz told NBC Chicago the group wants to make progress on remaining issues, however many there may be.

Union leader to Chicago teachers rally: In for the long haul

"We believe we're not that far apart and should be able to wrap these things up," Ruiz said.

Both sides admit some progress was made. The board agreed to the teachers' proposal on sick days, a health care rate freeze and seniority. They still haven't come to terms on a new teacher evaluation system, and Lewis said they remain "miles apart."

Follow NBCChicago.com's live blog on the teachers strike

Chicago’s school leaders are proposing that student performance on standardized tests count toward 25 percent of a teacher’s assessment, growing to 40 percent in five years.

But Lewis is critical of Emanuel’s push to make great use of standardized tests in teacher reviews, calling the process flawed. Union officials say the system wouldn’t do enough to take into account outside factors such as poverty, crime and homelessness.

"Evaluate us on what we do, not the lives of our children we do not control," Lewis said in announcing the strike. It was unclear what union officials proposed instead.

Chicago teachers strike affects 350,000 students

"This is far more than a labor struggle," American Teachers Federation President Randi Weingarten said during a big rally downtown -- the second massive protest this week. "This is a struggle for the heart and the soul of public education for the kids of Chicago."

In response to one potential next step being weighed by Emanuel and city leaders -- an injunction requiring teachers to get back to work -- Lewis said the mayor doesn't have the legal standing.

"We have a completely legal work stoppage, we have followed every rule," she said.

Meanwhile, student attendance at the 147 strike-designated schools and safe haven sites was reportedly low for the second day. At a South Side YMCA, the site saw just 35 kids Monday and Tuesday, lower than expected.

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M. Spencer Green / AP

Parents of Chicago public school students, Carmen Brownlee, left, and, Latonya Williams, right, walk a picket line outside Shoop Elementary School in support of striking CPS teachers, Sept. 11, 2012.

With negotiators trying to hammer out an agreement that would end Chicago’s teachers strike, one of the key sticking points is how to evaluate whether a teacher is doing a good job, an issue that has riled school boards across the U.S. in recent years.

Chicago’s school leaders are proposing that student performance on standardized tests count toward 25 percent of a teacher’s assessment, growing to 40 percent in five years, according to NBCChicago.com.

But Chicago Teachers Union President Karen Lewis is critical of Mayor Rahm Emanuel’s push to make great use of standardized tests in teacher reviews, calling the process flawed. Union officials say the system wouldn’t do enough to take into account outside factors such as poverty, crime and homelessness.

"Evaluate us on what we do, not the lives of our children we do not control," Lewis said in announcing the strike. It was unclear what union officials proposed instead.

The battle in Chicago over using student test scores to judge teachers is just one front in a nationwide battle over how to make sure teachers are doing a good job, and that taxpayer dollars and student time aren’t going to waste.

"This is going to become a long-term battle that everyone's watching very closely," said Eric Hanushek, a senior fellow in education at the Stanford University’s Hoover Institution, a conservative research center. "Teacher unions are at a crossroads: Are they going to participate in designing better teacher evaluations or resist and not change anything. The Chicago union seems to be taking the resist option, drawing their line in the sand."

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The Obama administration, through its $4 billion Race to the Top competition and waivers from the Bush-era No Child Left Behind, has urged states to change teacher assessments to make use of test data as a key component to set a teacher's pay or end their employment. The administration granted waivers to states that promised to show improvements in student and school performance and link teacher evaluations to student test scores.

Supporters say current review tools fail to give administrators a reliable assessment of a teacher's effectiveness, while critics argue there's no evidence linking student performance to a teacher's worth.

"Teacher evaluations should be based on multiple measures," said Marcus Mrowka, a spokesman for the American Federation of Teachers, which has 1.5 million members. "Testing has a role but should not sanction teachers but inform instruction."

Twenty-four states now require teacher evaluations based on some measure of student growth, according to an analysis by the National Council on Teacher Quality, a research and policy group. Public school districts in Tennessee and Washington, D.C., recently implemented new teacher evaluations tying outcomes to merit raises, while Colorado and New York are deep in the process of developing an evaluation system, the council noted.

In the past three years, at least 20 state legislatures have passed bills setting up new teacher evaluation processes, according to the council. Illinois joined the ranks last year when its legislature passed a law mandating new teacher evaluations, with Chicago’s leaders rushing to embrace the system, called the Performance Evaluation Review Act.

“The evaluation system should be built around continuing improvement of instruction,” said Rob Weil, AFT’s director of field programs and educational issues in Washington, D.C. “Evaluations should help people improve and we need to build systems that give teachers the information they need so they can improve. The process should not be punitive.”

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In Chicago, Lewis has warned that as many as 6,000 teachers could lose their jobs under the new evaluation system. The union represents about 25,000 teachers and staff, who walked off the job Monday.

School officials say they do not know how union leaders determined that number, and telephone calls by NBC News to union headquarters went unanswered Tuesday.

Emanuel has promised that teachers would not be fired in the first year of the evaluation process.

Union leaders, however, are still resisting.

“This is no way to measure the effectiveness of an educator,” said the union in a statement. “Further there are too many factors beyond our control which impact how well some students perform on standardized tests such as poverty, exposure to violence, homelessness, hunger and other social issues beyond our control.”

About 60 percent of students in Chicago public schools complete high school, according to the Illinois Policy Institute, a nonpartisan think tank. 

“We are spending more and more on students, throwing more and more money into the system,” said Ted Dabrowski, vice president of the Illinois Policy Institute. “If you want the best teachers in the system, then teachers should be paid and promoted based on their performance. It’s important that we improve the system, which has become a failed system.”

Do you have an education-related story? Contact Sevil Omer at sevil.omer@msnbc.com

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Sitthixay Ditthavong / AP

Members of the Chicago Teachers Union hold an informational picket outside Willa Cather Elementary School on Monday in an effort to call attention to ongoing contract talks with the city's Board of Education.

The Chicago Teacher's Union voted unanimously Thursday to strike on Sept. 10, with members agreeing to head to the picket lines unless a deal can be reached with the city’s schools -- a move that could affect 400,000 students.

The result of the ballot, cast at a Chinatown union hall, was met by cheers from the more than 700 delegates who voted.

The decision comes a day after CTU president Karen Lewis issued a 10-day strike notice. Sept. 10 is the earliest that teachers can strike, and the date falls a week after many students head back to class.

"We're tired of being bullied, belittled and betrayed," Lewis said at a news conference following the meeting. "We have done everything asked of us, yet we continue to be vilified and treated with disrespect."

Read the full story at NBCChicago.com

Lewis said negotiations will continue until a contract is hammered out that resolves concerns over wages, job security and a new evaluation system.

In a statement, Chicago Public Schools chief executive Jean-Claude Brizard said: “If our priority is our kids, then strike should never be an option.”

"Should CTU leaders decide to strike on September 10, more than 350,000 students will be taken out of their classroom, just as they're making progress with a Full School Day. Varsity sports for 11,000 student athletes will be suspended. More than 400,000 daily breakfasts and meals will no longer be served.

“College transcripts and recommendations for 20,000 seniors will be put on hold. If our priority is our kids, then strike should never be an option. That's why we need to take advantage of each of the next 11 days and work until we reach a fair resolution for our teachers that will allow our kids to stay in school where they belong."

CPS officials have been making preparations in the event of a strike.

Brizard said the district plans to keep 145 schools open for half-days even if the Chicago Teachers Union calls a strike and teachers walk off the job. The union on Wednesday filed a 10-day strike notice that would allow them to strike as early as Sept. 10, just a week into the school year for a majority of public-school students.

If that happens Brizard said CPS will partner with City Sister Agencies, local faith organizations and other non-profits to keep kids engaged.

"We need to be prepared to provide our students with services they need should CTU leaders decide to strike, and our Children First plan will do that," Brizard said.

As part of the precautionary strategy announced Thursday, CPS would keep the group of schools open from 8:30 a.m. to 12:30 p.m. Monday through Friday to provide activities to children and keep them engaged in independent reading, arts, sports and computer-based programming.

Per the plan, the schools would be staffed with Central Office personnel, non-CTU employees and organizations that submit a request for proposals to provide programming. Brizard said the staff-to-student ratio would be capped at 1 to 25.

CPS also said it will provide breakfast and lunch to all students at the locations and will extend between 70 and 80 Chicago Park District summer camps.

Brizard said he's committed to avoid a strike but says CPS will be ready if the teachers strike.

“These plans are simply a precaution," Brizard said, "but we have an obligation and responsibility to our children and their parents to make sure they are not left behind in the event of a strike.”

CPS said it is seeking a waiver from the Illinois High School Association to continue sports if teachers strike.

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New York power utility Consolidated Edison Inc locked out its unionized workers early on Sunday after contract talks broke down, both sides said, raising the possibility of power cuts during a summer heat wave.

The company asked to extend negotiations for two more weeks, it said, but the union, which had threatened a strike, refused. In response, the firm told union members not to report for work on Sunday.

Reuters reported that the action increased the risk of power outages if a continuing heat wave puts extra strain on the electrical grid for New York City and suburban Westchester county.

However, a utility official told the New York Daily News that customers should not expect to see any adverse effects.

"Both sides are far apart," said company spokesman Mike Clendenon. "We asked the union to extend the talks for two weeks but they refused."

"We can't operate the system reliably for customers if the union can still call a strike at a moment's notice," he said.

He did not use the term "lockout" but said the company notified unionized workers not to report for work. ConEd managers have been specially trained to handle emergency or maintenance work, he said.

John Melia, a spokesman for the Utilities Workers Union of America (UWUA) said that as of 2 a.m. Sunday (EDT) its 8,500 ConEd power workers were locked out.

"ConEd took the extreme measure of locking out its unionized workforce putting the city of New York and Westchester county in peril during a heat wave."

The lockout came as the summer's second heat wave hit the city of over 8 million people, with stifling temperatures near 100 degrees Fahrenheit (38 degrees Celsius), raising demand for power to operate air conditioners.

Both sides continued talking for over an hour after the midnight Saturday deadline expired, but failed to reach a settlement over a new contract for the company's unionized workers. A major sticking point in the contract was ConEd's plan to phase out defined pensions.

The union membership had authorized its leaders to call a strike at midnight on Saturday, when the collective bargaining agreement expired. A similar strike in 1983 lasted nine weeks, while a blackout in July 1977 - caused not by labor action but by lightning strikes - resulted in looting and civil disorder in the largest U.S. city.

As the deadline approached, 200-300 union members staged a rally in downtown Manhattan, chanting "If we go out, the lights go out."

Tony Ballone, a union delegate, told Reuters the main issues were pensions, wages and health care. "They (ConEd) want to take everything we have fought for 50 years."

"We're the first responders, we come out in rain and snow, we keep the lights on. All we want is a fair contract," he said.

With Con Edison workers locked out, company managers are left to fix whatever problems arise as New Yorkers crank up their air conditioners.

The utility had only just returned power to Brooklyn and other areas of the city blacked out in a heat wave 10 days ago. Still, with the lockout coming over a weekend, when many businesses in Manhattan are typically closed, demand for power will be lower than a weekday.

That would lessen the risk the utility will have to reduce voltage, commonly called a brown out, as the utility was forced to do last week in Brooklyn and Queens.

Still, the UWUA union stressed that without its skilled workers, the Big Apple could be facing outages if a deal is not agreed. Con Ed has 13,000 employees including union members.

Temperatures in New York City were expected to reach 92 degrees on Sunday and 90 degrees on Monday before slipping into the 80s on Tuesday before the Fourth of July holiday, according to AccuWeather.com. The normal high for this time of year is 83 degrees. 

Reuters contributed to this report.

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More than 200 transit police officers in south-eastern Pennsylvania have walked out on strike, forcing authorities to use private security firms to patrol subways and stops in the Philadelphia area.

Members of the Fraternal Order of Transit Police, Local 30 division, were called off their jobs on Wednesday, just minutes after negotiations with transit organization SEPTA broke down, NBC10 Philadelphia reported.

The station reported that the union has been in talks with SEPTA management from April last year and officers had been working since then without a contract.

A report in the Philadelphia Inquirer said union officials were demanding a $0.50 hourly pay raise to cover the cost of training certification for the 219 officers.

"At no time did we want to have to go on strike, but SEPTA forced our hand to do what we had to do right now," Rich Neal, of the union, told NBC10.

Read the full story on NBC10

In a statement issued on its website, SEPTA said that it did "not anticipate this labor action will affect any transit operations or service."

"SEPTA has hired a private firm to provide security at our major transit facilities," SEPTA said. "We have an agreement with the Philadelphia Police Department to immediately begin periodic checks of our stations and vehicles 24 hours a day, with an increase presence at our key stations during school dismissal between 2:00 p.m. and 6:00 p.m. and during rush hour."

NBC News and msnbc.com staff contributed to this report.