If the child was endangering or assaulting other kids or staff, the teacher can restrain them as per the standards of the school board (which it should have trained all staff or at least teachers on).
If it isn’t in this context, it is assault, plain and simple, no matter how chutzpadik or disruptive the klid might have been.
If its a one off, its still assault, but my understanding of the word abuse in the context of schools would indicate something that is repeated or going on for a long time. (I stand to be corrected. Its just semantics about the meaning and use of the word).
For a mandated reporter it has to be reported immediately. For anyone else it is simply witnessing a crime against a child. what would you do if it was your child?
And if all other considerations of common sense do not move you, I can’t think of a more appropriate application of dina demalchusa dina than the protection of the vulnerable from harm.