On a sunny day in Grapevine, Texas, three drones are buzzing around the head of a test dummy balanced on a pedestal. It's part of a demonstration outside the National School Safety Conference.

"We use drones to stop school shootings," says Justin Marston, the CEO of Campus Guardian Angel, the company selling the drones. In the event of a shooting, remote pilots fly the drones, housed at the school, at the shooter. They shoot pepper balls and run the drones into the shooter to debilitate them.

The technology is one example on a long list of products schools can buy to deter a shooter.

There have been more than 400 school shootings since Columbine in 1999, according to an analysis by The Washington Post. The latest was last month, when a former student opened fire at a Catholic school in

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