1. teachers not adapting to new times. In ye olde days, teachers had the attention of the students basically guaranteed (as the only thing students could do instead of paying attention to the teacher was drawing messages on papers and passing them around). That frontal instruction is bullshit and should be a thing of the past could be ignored. Now, however, teachers have to compete with the Internet for the attention of the students - but instead of embracing the change and throw some rotten traditions over board, they ban phones. LOL.
2. Mobbing/cyberbullying. I can understand that schools want to tackle this problem, but banning smartphones is a symbolic measure. The students will still use closed Whatsapp/FB/whatever groups after school, without the school being able to interfere in any way. Proper prevention measures, like teaching the students what the effects of social media can be, provide counseling, social workers, etc. - all that costs money, a lot of it. And so politicians rather ban smartphones so that the (rightfully) concerned parents shut up without having to spend huge sums of money.