Yes you're being quite naive. I can see you've said you were in the US, but there's been a growing and growing campaign to have Smartphone Free Childhoods etc etc, and lots of debate about how schools should ban phones. And surely they wouldn't be having that debate unless there was a problem?
This article is good https://www.thetimes.com/article/1100fdd2-79b1-4dc5-88e6-de87d2319294?shareToken=9ebc7793f3d212ca7aaea87174044798 (hope the share token works)
I'm in London too and a local academy trust bans smartphones (kids are expected to have a brick or nothing, but most have a smartphone at home). Other state schools are less good at banning - trying to say phones must be off, not seen, not heard, but the consequences are much less (given back at end of the day, whereas academy trust gives them back at the end of the term).
FWIW I think the issue is about social media use, and parents not understanding what their kids are up to, but that's for another thread! Also not having rules about screen time. Like the Sunday Times experiment, look up Panorama on the BBC, Can We Live Without Our Phones - worth watching.