To cut off students from the outside world / their parents
You seriously think the ban on phones in school has anything to do with an absurd attempt to 'cut off students from the outside world' when they go home every night? That doesn't strike you as absurd in the least?
Mobile phones are a distraction in classes. They're banned for the same reason game boys were, and then some. At least game boys were a passive distraction -- a way for a student to distract themselves if they indulges. Phones are actively distracting. Ring tones, notifications, chirps, pings, alarms, all serve to distract not just the student, but the entire classroom.