I think a lot of comments are thinking about this as a binary, or that it's actually about if children should have phones or not.

As an adult, there are plenty of places I'm not allowed to take my phone, or at least must completely power it off. Concerts, court rooms, libraries, hospitals...

These rules are there only because it disturbs others if my phone rings or I'm talking.

That applies to schools, too, but even moreso. It also opens the door even wider to cheating in various ways.

While it is important to try to teach kids to regulate themselves, the fact is that there are still frequent phones going off at concerts and annoying people talking loudly in libraries. Schools are much more serious in nature - it can affect test scores, for example.

It's also naive to think that all students / schools can just be taught to make responsible decisions. Many schools, especially inner city ones, are complete disaster areas. It's hard enough as it is to get the kids to even stop talking, or sit down, or stop assaulting others.