When smartphones first started coming out a high school teacher took mine away - there was no blanket ban but I had undiagnosed ADHD and I wasn't paying attention during class. As she was taking it I told her if it got broken while it was out of my hands that was her responsibility, it cost a thousand dollars. I wasn't a rich kid and I got it on a contract with the phone company. I remember she got really stressed out and cried about it during class.
If you multiply that by 30 kids in a class, conservatively, a teacher could be stuck sitting on 30 confiscated iPhones. That's like half their annual salary in kids claiming they broke their phone. Not to mention any claims that a teacher used a kid's phone for some nefarious purpose.