I think the officers did the right thing by handcuffing her -- this child was obviously out of control and she could have caused some real harm, not only to others, but to herself as well. As it was, the tape showed her punching someone (the assistant principal or whoever). "Arresting" her and putting her in a police car -- well, I think that might have been over the top. But restraining the child I think was the best anyone could have done under the circumstances.
The other thing is: We don't know whether this child has some physical or emotional problem that caused her to act out. I could only hope her parent(s) will look into that possibility, if he/she/they have not already done so.
She hasn't, believe me. The mother is part of the problem.