The thing is: smartphones exist. The young adults these children will become will live in a world where smartphones are an essential part of their life. Using a smartphone
is a practical skill.
That's why I don't think banning smartphones is the best idea. It is probably better than unrestricted access, but I feel that school should teach how to use them well instead. It is a bit like with calculators, there are classes with calculators, classes without, and classes that teach how to work with them, their strengths and shortcomings.
I don't know how to do it in practice though. Airplane mode and offline educative apps may be a start.