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.