I tend to usually side with the police, but in this situation that officer should be fired.
The kids were not listening, and were acting like pricks.. but they are kids. You need to be a bigger man in that situation..
I understand you need to listen to the police, and ya, that girl probably deserves to have her head slammed in the sidewalk, but that isnt the police's job. That guy seemed like a power hungry prick. The two other officers there seemed fine, just that one.
The kids really didnt pose much of a threat other than being a-holes.