At most, if kids phones were registered as "student" phones and registered to a school so that between certain hours, the phones allowed policies from the school to be applied to a phone while the phone is on the premises. Teachers could just disable them during class or allow for exceptions like if such and such kid was waiting for a critical call from a parent or something.. Classrooms could have beacons telling the phones they're in a classroom so if they go in the halls the phones could work... Or not.. Either way, there should be tech solutions to tech problems and teachers have enough to deal with, they shouldn't be further strained by having to police students always trying to find ways to sneak on to their devices.
Soon teachers might be a thing of the past and kids will just interact with an AI teacher anyway... The AI will be responsible for keeping them engaged and make sure they understand the material. At that point the AI can just shock them into submission if they whip out their phones... Muahahhaa... jk jk.