In one sense, the impulse driving these preparations is understandable. The prospect of mass murder in a classroom is intolerable, and good-faith proposals for preventing school shootings should be treated with respect. But the current mode of instead preparing kids for such events is likely to be psychologically damaging. See, for instance, the parting letter a 12-year-old boy wrote his parents during a lockdown at a school in Charlotte, North Carolina, following what turned out to be a bogus threat: “I am so sorry for anything I have done, the trouble I have caused,” he scribbled. “Right now I’m scared...