KIPP New Harmony and Ujima Village Academies in Baltimore were locked down on Thursday after students misidentified a man with a tripod for an intruder with a gun. (Baltimore Sun video)

Amid reports of an "active shooter" on campus Monday, Stevenson University students threw their desks against the classroom door, then prayed with near-strangers under computer carts.

Just three days later, elementary and middle school students at KIPP charter schools in Baltimore were hiding with their teachers in a classroom, while police searched the building for a gunman and hundreds of parents rushed to a nearby school to wait anxiously for word about their children.

There was no real danger to students or teachers in either incident. At Stevenson, someone had seen kids with a pellet gun hunting near the wooded campus. At the KIPP schools, a child saw a man with a photographer's tripod and thought it was a gun.

But in these times, when the names of schools from Columbine to Sandy Hook to Perry Hall have come to stand for unimaginable acts of violence, the public has a heightened sense of fear and vigilance, and a feeling that perhaps schoolhouses aren't safe places.

"Our shared cultural experience now includes such traumatic incidents," said Davis Shingleton, a psychiatrist at Hannah More School in Reisterstown. "We have to face reality, and the reality is that these things have occurred and could occur. That knowledge causes anxiety and hypervigilance, which must be managed."

Some students and parents criticize school officials for overreacting by sending alarming messages that turned out to be unfounded or for not sending messages until hours into the event. But safety experts and police say it is appropriate for schools and police to react quickly, as they did in both these cases.

"I think anytime in the months after a high-profile national tragedy, there's always heightened awareness, heightened sensitivity erring on the side of safety," said Kenneth Trump, president of the National School Safety and Security Services in Cleveland. "Oftentimes we see overreaction. You see that heightened response. That's OK. You want people to respond. We want threats to be treated seriously."

Baltimore police Commissioner Anthony W. Batts said that about 9 a.m. Thursday, school police received a report of an armed person on the KIPP school grounds in Northwest Baltimore. He said he is glad the students made the report.

"They saw something they misconstrued or thought was something of danger. They did the right thing: They reported it and we responded to it. And I don't think anything has gone wrong here. I think everything has gone very well," Batts said.

What students had seen was entirely innocent. Two University of Maryland, College Park journalism students had brought a tripod to the KIPP campus to record video for a class. They had permission from both the school and the school system to be on campus, according to Lucy Dalglish, dean of the Philip Merrill College of Journalism at College Park. City police confirmed that a man with a tripod was mistaken for a gunman.

Tamaya Evans, a KIPP sixth-grader, thought she was just practicing a drill when the school went on lockdown. She and nearly every student in the nation learns to lock the door, push desks in front of the door, and hide with their classmates and teacher.

Tamaya said she and 26 classmates moved to the back of the room. The students sat for more than an hour and a half, passing the time by thumb-wrestling and playing quiet games under the supervision of their teacher.

"At first it was like, 'This isn't real,'" Tamaya said. "'This isn't really happening or they would say something on the intercom.'"

Then, as the wait grew longer and longer, "reality kicked in," the 12-year-old said.

"I was scared; I was crying," she said. "I started thinking about my family. What if something happens and I don't see them?"

She remembered saying goodbye and "I love you" to her parents as she left for school Thursday morning. "But that didn't seem like enough," she added.

Hundreds of parents gathered in the parking lot at Polytechnic Institute, where KIPP brought the students. Lalishia Connor said she was in shock. She said she had a niece who attended Sandy Hook Elementary in Newtown, Conn., where a mass shooting occurred in 2012, so she knew it would be a long day.

"This is a process, and this is going to be a while," she said. "All we can do is sit and wait and pray that our babies are fine."

The lockdowns now practiced by most schools in the country help prepare students psychologically because students know they are doing what will make them feel safe during a tense situation.

Sharon Stephan, co-director of the National Center for School Mental Health at the University of Maryland, said that the anxiety and intense feelings children and parents have after a lockdown is a natural and normal response.