I'm surprised that you're surprised, OP. It is completely the norm for 12yos to have smartphones these days.
I'm guessing that a lot of parents who choose the type of school that you describe tend to be quite conservative anyway, so perhaps more likely to go along with the school's strict phone policy. But yes, it's definitely a fairly unusual bubble that your kids are in.
Personally, I sit on the side of thinking that it's better to teach kids to interact with technology in a safe and sensible way, rather than banning it, but if it's the norm for that school, then at least the kids won't feel like they're missing out in any way.
I think it's much, much harder for the tiny minority of kids in mainstream schools who may find that they're the only one without a phone and therefore end up being excluded from group chats and social arrangements etc. I understand parents' concerns about smartphones etc but will never understand why parents would choose to put their kids in that position.