I think this would be reasonable except for all the billion dollar companies pouring their money into researching how to keep the kids on their phones. The solution supposes that there's a state of mind the child could have that would give a smaller intrinsic need for the phone and that would be that, but this falls down when there are better funded and more powerful entities than the schools and parents pushing extrinsic motivations to stay on the phone for longer and longer.

I remember being a kid and I wouldn’t need any reason to using smartphones at school. And we did bring banned things like walkman, gameboy, dumb phones,… And we only needed to spend 5 hours there.

What I hear from teachers is they are constantly bombarded by notifications. They aren't idly playing on their phones. They're constantly responding to things happening.

There's seriously no alternative to banning them, and I don't even see the interest in trying.