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Thirty-nine states require lockdown, active-shooter or similar safety drills. Other states have less explicit requirements or leave it to districts, according to the Education Commission of the States. A Mississippi task force has proposed twiceyearly active-shooter drills.

But even as the drills become routine for many of the nation’s 51 million elementary and secondary public school students, there is no consensus on how they should be conducted, experts said. No data exists, for example, to show whether a drill with simulated gunfire is more effective or whether an exercise that’s been announced in advance is taken less seriously than a surprise.