I do not see an assault here, sorry.
Let's say you went to a cafe and for whatever reason the waitress told you you had to leave. You refused to get up and leave. Then the manager comes and tells you the same thing. You still won't budge. Then they call police and the officer tells you to leave of you'd be arrested. You still refuse so you get removed forcibly and arrested. That isn't assault either. The reason this looks bad on the surface is the small stature of the girl (but small stature doesn't imply she was in the right and the cop in the wrong) and the silly chair-desk combo so common in schools that made it harder for the cop to remove her.
Agree.
Any attempt to remove her would have resulted in the same thing. Physical distress which will allow her and her family to successfully sue the school and city and make kids in school even more unruly, uncontrollable and untouchable.
And end up rewarding her behavior.
If she was 18, he was justified in physically hauling her off. She's an adult, not a child.