> Once enough students tip over to "I'm going to use it and I don't care", the school can't do anything about it.

The school can totally do something about that, they just have to be willing to confront the student and enforce the rules.

I don't know your background, but this definitely reads like someone who either isn't, or doesn't have a family member or friend as faculty in a public high school. You enforce the rule, and confiscate the device. You have an irate parent now emailing you constantly, coming in and demanding that their child be able to have their several hundred dollar smartphone back, to which they will hand it back to their child, give them a weak "don't do that again" at best, and likely forget the incident ever happened.

A school can't have rules if they're that afraid of one parent complaining. They're too weak.

What if the student beats other kids up, and the parent behaves similarly in response to discipline? Should the school tolerate that defiance, too?

The school obviously can't take the phone and throw it in a shredder or keep it for the year, but they can and should be able to keep it for the school day (and every other school day, if need be).