Well, by their teenage years, why not all the reasons adults need smartphones fully accessible? Looking up information from authoritative sources? Emergency contact? Coordinating schedules for office hours?

Schools often simultaneously demand more from children than workplaces do adults, and give them less opportunity to excel.

I'm not saying work-inappropriate phone use should be accepted, but taking them away entirely is downright irresponsible. Just like schools who still demand students write on a notebook instead of using a laptop. Raise your hand if you had RSI-related issues for a decade or more after high school? We old people tend to forget how bad school used to be (and can be) for physical and mental health AND for learning.