A high school in the far northwest suburbs of Chicago was locked down for about 20 minutes on Wednesday afternoon after the sound of two gunshots rang through the halls.

But the shooting -- done with blanks -- was all part of a drill to educate students and staffers at Cary-Grove High School in Cary, Ill., on what to do in the event of a real emergency. Cary is located about 45 miles northwest from downtown Chicago.

"For fires through fire drills, we prepare them for tornadoes through tornado drills and unfortunately a reality of today is that we also need to prepare them for other safety concerns that are more immediate," said district 155 spokesman Jeff Puma.

Police and administrators did a complete sweep of the building after two shots were fired at opposite ends of the school. Students were ushered to the corners of their classrooms as if a gunman were roaming school grounds.

Reaction from students was mixed.

"We learned, like, where to go and stuff, so that was helpful," said one student.

"I feel like they could have done it better. Honestly, more blanks around the school because for the most part you really couldn't hear it," said another.

Also on NBCChicago.com: Teen girl among 3 dead, 8 wounded in Tuesday shootings

Despite the difference of opinions, administrators and police said the information learned could be life-saving.

"The idea was to allow our students to have something in their head of what it might sound like in order to react more quickly," said Puma.

Many parents notified of the "code red simulation" earlier this week said the simulated gunfire was too strong a tactic.

"It's sad, but the reality is these things happen," said Cary police Chief Steven Casstevens.

A district official said Wednesday's drill wasn't the first conducted in the wake of the Sandy Hook school shooting. The first was done just four days after the Newtown, Conn., tragedy.

Previous story from NBCChicago.com: Parents uneasy about simulated school shooting

A Texas man is facing third-degree felony charges of making a terroristic threat after he allegedly told elementary school staffers he brought a gun to the building, NBCDFW.com reported.

Officials say Ronald Miller was unarmed Wednesday when he told a school greeter outside Celina Elementary School that he had a gun, according to NBCDFW.com. The town of Celina is just north of Dallas.

The greeter froze in panic when Miller said he was a gunman and his target was inside, Celina Independent School District Superintendent Donny O'Dell told NBCDFW.com. Miller was then able to walk into the school and entered the office.

"He told them that he is a shooter and 'you're dead, and you're dead,'" O'Dell told NBCDFW.com. Never showing a weapon, Miller then reportedly revealed his stunt was a test of school safety and he wanted to talk to the principal.

School staffers knew Miller, who was a father of a student, and police were not called until he left the school, The Dallas Morning News reported. He was arrested Wednesday evening and is being held in lieu of $75,000 bail, the newspaper added.

School security and gun control have been hotly debated since the Dec. 14 shooting at Sandy Hook Elementary School in Newtown, Conn., claimed the lives of 20 children and six adult staffers.

In a letter to parents dated Thursday, O'Dell said Wednesday's test was done "in a rogue manner."

"We have always had a security plan in place that involved our police officials," O'Dell wrote. "However, because of recent events we have ramped-up our security efforts on all campuses."

O'Dell did not respond to NBC News' phone and email requests for comment Friday. Representatives of the Celina PTA board did not respond to email requests for comment Friday.

David Siano, a parent at the school, told NBCDFW.com that the incident shows that "we are not prepared."

"His intent was just simply to say, 'you've done nothing' and that's what it showed," Siano said. "So (if) that’s what it takes, it’s a shame."

Another parent Misti Schramme told The Dallas Morning News she trusts security measures in Celina and thinks her child's school is safe: "You can’t live in fear all the time."

School safety expert Ken Trump told NBC News on Friday that he encourages parents to "ask probing questions" about their child's school security and emergency prep.

But he advises: "Don’t go off the deep end to be overly dramatic." Instead, Trump recommended that parents choose avenues like scheduling an appointment with the principal, attending safety or crisis team meetings at the school, or going to the school's PTA.

In the last few decades, Americans have witnessed a number of high-profile school shootings, including the 2007 attack at Virginia Tech and the 1999 massacre at Columbine High School in Colorado.

On Thursday, an armed student entered a Taft, Calif. high school and wounded a 16-year-old teen. A teacher and campus supervisor persuaded the shooter to drop the gun.

NBCDFW.com's Catherine Ross contributed to this story.

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