It must be difficult for the teacher, if the "victim" won't give their side of the story, to do a great deal with no testimony/evidence of what actually happened. To always side with the accuser would give a loophole for getting at ones fellow pupils. Keeping an eye on things, and letting the older girl it isn't acceptable, is probably a decent choice of action.
If things are really bad maybe your daughter could change friends. It is good for her to be able to physically react and make it clear she will tolerate no messing, but if her personality is not like that any action is likely to be half hearted and cause more problems than it solves. Stay away from those unable to behave properly, choose friends with care.