The kids could also just ditch their phones and learn to live without them for a few hours when they wanna leave campus. Or buy a cheapo used one and use WhatsApp/Messenger off WiFi if they need to communicate with friends.
Not that I'm saying the parents doing this are in the right, just that it's pretty easily defeated IMO. Kids have been sneaking around behind their parents back since the beginning of civilization doing things they wouldn't approve of, this is just the next step really