This ban doesn't protect them from regular in-person bullying.
Just crowd-sourced, Internet scale bullying.
Ever had your photo released mockingly, and been laughed at by an entire school? Or even millions?
How does a phone ban stop this from happening? Not to mention the challenge of actually enforcing it. I suspect the kids who want to take pictures of others and upload them mockingly will still do it.
True, but it does make it much harder to do while at school (if enforced passably), which is likely when much of it occurs already.