Turning off Group messages on child's iPad

Our school is currently having an issue with some kids adding other kids to group chats on iMessage without their consent. When they try to leave the group it re-adds them when someone sends a new message? I've tried to do it under Settings/Messages, but the group messages toggle is not there.

I've also got Screen Time enabled on the iPad, but there is nowhere where this can be restricted.

Can someone please help because this is a potential Cyber Bullying risk as the kids who create the groups sometimes do so to say bad things about other kids, and then everyone in the group will be implicated in the issues, even though they didn't want to be there in the first place.

Might be something that Apple can add to the Screen Time portion of security.