To better understand how students and staff are taught to respond to active-shooter situations, USA TODAY contacted every state and the 25 largest school districts. Active-shooter trainings vary widely, lack oversight and may do more harm than good. At least 42 states require schools to run some form of drill that could address an active-shooter situation. Although a student in Missouri may never experience an active-shooter drill, students in some Florida counties have them monthly. Drills can have negative mental health impacts for students, teachers and families, but researchers say it’s hard to study whether drills help save lives. Scroll through to learn more about active shooter drills across the U.S., and click the link in our bio to learn more.