It can be challenging to make your way to the school’s office once the facility has been taken over by an armed gunman, which is a great reason for kids to have comms at all times, if not one that commonly arises.
Well, in order to have _any_ children calling out, at least one of them would have to have a phone. One great reason for not banning phones is to increase the chances that at least one student has one.
We don't even have to get into how some students would still have phones, because of all the teacher and staff phones and wired phones in almost every room.
Even in the shooter situation, there's no need for student phones.