When my daughter was in jr. high she was having similar problems with bullies on the bus. The difference was that they were bothering someone else and not her, she was just standing up for the other girl. I backed her up 100% and said don't be afraid to do the right thing - don't let someone do that and just stand by and watch, keep up the good work. I also let the principle know I told her that and I would have to pursue it further if anything happened discipline wise or injury. They needed to stop the root cause. In this case it seemed to work out eventually.