There are a lot of things children are not allowed in school. Why are phones, which at best have little to no benefit, and at worst may impede learning, cause depression, cause bullying that follows the child all the way back home from the playground, cause social stratification, etc allowed until we have a better idea of their impact?
There's data to suggest that mobile phone use and social media in particular are responsible for vastly higher levels of mental illness than any bullying has ever been.
Cutting humans off from reality is an extremely bad solution to bullying. We can and need to do better.
Carrying a phone just because of potential school shootings is like wearing body armor for the same purposes.
Phones cause depression? More likely to be the abilities of the phone will allow people to access certain things that could cause depression. It's a point I'll make soley as it reminds me of "ice caused the accident" argument, and why uk cops call Road Traffic Accidents Road Traffic Collisions.