Hello, permits and the like? Plus the fact that some of these cherubs were hurling concrete at John Law.
If they're assembling without a permit, the police can read them the riot act and tell them to disperse - they can't just block everyone from leaving the school.
If they're hurling concrete the police can either arrest the hurlers or order the mob to disperse or both. They can't block the school exits. I can understand police doing that if the safety of the public or the students would be at risk (for example, in a lockdown situation) but that doesn't apply here - everyone just wanted to leave to walk to a protest.
I'd like to know what the cops were doing there in the first place blocking the exits - it looks like they're the cause of all the trouble.
The school is free to enforce their rules by telling everyone that "if you leave, we will expel you" or something like that. The police have no business blocking school exits.