And it's obviously a big problem for adults as well as kids, but I think there are many nuances there. One would be that we generally accept adults engaging in self destructive behaviors as part of the liberty of being an adult. You're free to go gamble away your life savings as an adult - but you won't even get in the door as a kid.
Another is that lot of the undesirable things that happen in schools (gossiping, conspiracies like false reporting, bullying, in-out crowds, image crafting, etc) get greatly magnified when you introduce social media. And that would happen even if the most benign/benevolent social media, absent any 'algorithm', engagement maximizing, and so on.