This is what I would do...
Go to the school, find this "gang" (I hate when kids try and act tough), and threaten them. Say if your daughter is touched, you will beat the ever living sh-- out of them. Of course, if you do that, make sure there are no witnesses.
The other approach would be to threaten a lawsuit. There is nothing more a school district hates than bad publicity/losing money.
Good luck. Keep us updated.