No mobile phone at schools?

I can see some merits in this. Students often have them switched on in class and that is disruptive. Also some students like to prank each other with them. I also know of instances of where things have been photographed or filmed with a phone and they end of on u tube or elsewhere on line and having access to phones at school allows this type of bullying to be perpetuated. Most schools have phones available for use so why are mobiles needed?