No they weren't. There was no threat at all of any kind of emergency situation being created. All indications were that all the students were going to leave to go to join a protest (and no doubt many if not most would just use the protest as an excuse to leave school early and just go home or wherever).
Everyone would be free to leave after school hours anyways. What's the difference if they get out a couple hours early? There was no reason that the school couldn't have just handled this as just kids skipping class, which is all that it was.
You should have stated at the outset that you were there and had such a command of the situation.
I only know what I read from the local newspaper articles. I'm speculating based on those - if I'm wrong about something I'd like to be corrected. (Seriously. This story, as I read about it, just seems outrageous to me. Maybe I'm making some false assumptions; maybe there's some info I'm missing which would make the police action understandable.)
I think you are. But I could be wrong. The fact is that I trust the police in these situations rather than a mob of kids. My belief will come up and bite me in the ass occasionally but stands me in good stead most of the time.