Studies of whether active-shooter drills actually prevent harm are all but impossible. Case studies are difficult to parse. In Parkland, for example, the site of the recent shooting, Marjory Stoneman Douglas High School, had an active-shooter drill just last month. The shooter had been through such drills. Purposely countering them may have been a reason that, as he was beginning his rampage, the shooter pulled a fire alarm.

In any case, preparedness drills always change the baseline level of risk that people perceive. This heightening can manifest as stress and anxiety, not to mention changing the way kids understand how people treat one another—to even consider violence an option, not in some abstract way.

Colleen Derkatch, an associate professor at Ryerson University in Toronto, studies how we assess risk when it comes to our health. “The more prepared we are, the more heightened our sense of risk,” she told me. “And one potential effect we haven’t considered is how these kinds of preparedness activities affect kids psychologically, and could increase a sense of feeling at risk. They really expand the ways in which we feel increasingly under siege.”