"The longer you can prolong this person's means to get access to you, the better chance of getting through this you have," he says.
An ability to survive this kind of attack also depends greatly on planning ahead. So in training teachers and school officials, Strategos tells its students to always begin with the classroom door closed and locked.
In the case of the Newtown shooting, parents' first reactions may have been to rush to the school to ensure their child is safe. Flooding the streets with traffic will only make it more difficult for first responders to get to the scene. A school system should set up a series of locations where parents could wait for more information.
A reverse 911 dialing system can also help disseminate information, Warren says.
Ideally, a school system would set up four separate established locations. This would allow a place for a command center, media outlets, a safe zone for children and meeting place for parents. Having multiple locations would also reduce the chance of a secondary attack if the shooter were familiar with the plan.
"If we'd done our job, we would have educated parents on community response," says Warren of how communities can best prepare. He also stresses that it must be up to the community as a whole to prepare, to ensure a seamless understanding between law enforcement, schools, parents and others who might be involved in a tragedy like this.
"It has to be a community response—community preparation," he says. "It's not if it happens here, it's when it happens here."
Do not take instructions from anyone except proven law enforcement, or someone you know
Never unlock a door for anyone unless you are sure that person is actually a law enforcement officer, or you hear someone you know well enough to tell if they are under duress.
When directing groups of people, particularly students, make sure their hands are exposed, ideally over their heads, Warren says. This makes it more difficult for a shooter to blend in with the crowd.
Act on any clues than an attack might happen
The shooters involved in Columbine and Aurora could have been prevented, either by parents or friends who acted on threats of bringing a gun into the school, or passersby in the parking lot who questioned the man entering the theater's side door.
Call the police, Warren suggests, and have them verify whether there is a threat.
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Paul D. Shinkman is a national security reporter for U.S. News & World Report. You can follow him on Twitter or reach him at email@example.com.