This seems reasonable to me.
But if you want the outline of why this sort of thing shouldn't be a law, the issue here is that it might be that the best educators will ban devices. It might be that the best educators encourage and leverage devices. Or it might even be that they take no position on the matter. Any which way, either there is consensus and the Californian legislature doesn't need to act because the schools can, or there is no consensus and the Californian legislature should leave it to parents and educators to sort out without pretending it has some sort of competence to add anything to the mix.
Legislating everything that seems like a good idea at the time is a pretty safe way to end up doing real damage as people initially had many options to act cleverly but now do not. This is not something that the politicians have any reason to be dealing with.
However, this bill doesn't appear to specify how phones are restricted, just that there is a school policy. Further it gives protections to students who need it. This is why I find it reasonable.
Schools still appear to have a choice in implementation. That said I'm not a Californian, I'm childless, and I only scanned through the bill.