Present your driver’s license to be scanned and verified. Have your photograph taken. Pass your belongings through a metal detector. Welcome to your child’s school.

Twenty years after the Columbine High School shooting, a school visit can feel like going to the airport.

See-through backpacks and armed officers are common sights on campus. So are “run, hide, fight” trainings, full of tips on how to survive an active shooter. Some days might bring lockdown drills that students are not told in advance are rehearsals, not real threats. And in rare cases, the adult teaching algebra or social studies might be armed.

Hundreds of millions of public dollars have been spent “hardening” schools against every parent’s nightmare: something like the 1999 shooting in which two students killed 12 of their classmates and a teacher at a Colorado high school, and set off an enduring debate on gun violence and the safety of the nation’s schools.