I've heard that it's needed to counter heavy parent opposition. If it's not enshrined higher, the PTA will not hear the end of it.

> Local school policy is good enough

An angry parent that can't possibly imagine life without immediate access to their kid throughout the school day is likely to respond differently to the statement "This kid can't have their phone on during the day." when the reason is "local school policy" than when the reason is "state law".

In the former situation, a sufficiently-offended parent will likely try to get the administrator in question fired (or -at minimum- make their life dreadfully difficult). In the latter, the only people the parent can really appeal to are the folks in Sacramento. Busybodies like that rarely have the clout to do anything meaningful to the governor or his staff.

Anyway. It's quite possible that local school administrators requested that the governor help them out by making it so they can say "Sorry, I'd love to help, but The State says I have to order teachers to ban phones from classrooms. You know how it is. Take it up with the governor if you're mad about it.".

If the law mandates attendance, then the law should regulate what that attendance entails. Much like how state or municipal exclusivity agreements on broadband should regulate how those broadband companies should behave toward their customers. With great power comes great responsibility.

As for private schools, yes, they should be left to their own policies. But I haven't seen anyone suggesting otherwise.