They are where I live, both elementary and secondary schools. The first results are promising; kids have more concentration, interact socially with each other more, etc.

I mean if it's a school where they also have to carry laptops and use digital schedules I don't think it makes a difference, but it's a good first step.

One issue was that each phone also has a camera, so people would seek out / make trouble on purpose, spy on people and post it online, etc.