The school uses Yondr pouches: https://www.overyondr.com/phone-locking-pouch

It's a locked bag you put your phone in, but then you keep your phone (that you can't access) and they unlock it as you leave. It's popular for concerts and stuff too.

Interesting phenomena: kids will find an old phone, charge it up, and then that phone gets locked in the Yondr pouch (keeping their real phone unlocked). So now they have to spot check phones for cell service when they lock them.