The problem is that you are treating it as a thing to forbid. No, that never works. Kids are bound to rebel against some ultimatim of "you can't do X because it's bad or you!". That's not a satisfactory explanation for an ineivtably inquisitive adolescent. Use it as an opportunity to communicate and teach them t self-moderate. Definitely enforce it at home as well, but if you can't trust your kid outside your watch you're missing the point.

Moreover, they spend a lot of time at school, but not all of it. If teachers can't keep kids of social media for hours on end that's a problem with the school, not the social media. They banned access to phones in my school outside of emergencies and kids passed times playing video games on a TI-84. No way in heck any kid in my school back in the day would get more than a few messages between teachers peeking in