I've watched this video several times now. I certainly don't endorse police brutality, but I don't think this qualifies at all. ~She was trying to flee the scene. I believe, based on the video, she ignored the policeman telling her to stop. He didn't bash her head against the wall; he pushed her so she would fall down, and then tackled her to keep her down. It looks like her head may have hit the wall, but he did not purposely bang it against the wall. It also doesn't look like it hit the wall hard enough to concuss her. Had she not been trying to flee the scene, he would not have had to do anything violent with her at all. She is the violent one: she is the one who was in a street fight, and based on the part of the video where they were fighting, she was the agressor, at least during the part of the video that shows the fight.
I've seen enough of these teenagers who are aggressive and violent to know they are frightening and scary; I would not want to have to deal with one who was out of control. I expect the cop has seen far more of this type of behavior from people, both male and female, of that age than I have. They are not nice kids; they are capable of great violence. They are often bigger than the previous generations: this girl, at 15, is nearly as big as the male cop. He is within his rights to do what he did to stop someone fleeing the scene of a violent altercation.