You must have missed the part where cops are trained to pick up students while still in their desks and flip them over backwards onto the ground.
[was waiting for the "he's just doing what he's trained" excuse to come up...]
He didnt flip her backward, the desk flipped because she wouldnt let go while she was resisting arrest.
The same thing in most of these cases: first refusal to comply with a lawful order, then resisting arrest.