If it is a school rule, and this just my opinion – of COURSE it is proper. If there is reason to suspect that bochurim are not complying with the rule, then the principal is within his rights IMO. And I am the one who generally feels that at times there are too many rules being imposed on our kids. Nevertheless, if you send your child to a school that specifies the rule of no cellphones, they have a right to enforce that rule. What if he thought the kids were smoking pot? Wouldn’t you agree he would have the right to randomly check on them by asking them to empty their pockets or whatever else he wanted to check (lockers, book bags, etc.)? A Yeshivah, we tend to forget, is NOT a Democracy.
I do feel, though that there should be some availability for a bochur to have a cell phone (but perhaps give it to the Rebbie at the start of the day, so the boy doesn’t have it on him, to text or for other shtuss, but it is available in an emergency. The old line, “Well if the parents need to reach the student they can always call the school,” doesn’t work. Most schools have voicemail menus that take forever to navigate. In an emergency Chalilah, it could waste very precious time. There is surely a way to compromise.