Been there and bought the t-shirt.
My son was bullied by some boys in his class. I knew the ring leaders parents socially but I initially went to the school. The school pulled in half the class and had enough evidence to bring the parents of the boys in. I then had ringleader brought round to the house by his Dad with profuse apologies and also then had the boys mothers do the same in the playground. All were insistent that if anything happened again, I should go straight to them. I had the women in tears with apologies.
Then, a few weeks later I witnessed the boys at it again, to be fair, it was through a window so I couldn't hear what the boys had said, but I could see the aggressive stance and they were obv shouting right in my son's face and he came out upset.
I went to school again but also spoke to the parents - this time they weren't interested and basically said that it was my son's own fault and that the boys had just been singing!! I ended up in quite a nasty intimidatory position with the boys father - formerly our "friends"
Anyhoo, the school were pretty good at keeping an eye on it, but my son ended up being tested for aspergers as he was so anxious while in school and 3 and half years down the line he still gets bother from the class and we have taken the decision to put him in a different high School to the rest.
I would say you could get any reaction if you speak to the parent, my view would be to speak to the school and let them sort it out and if you are approached (unlikely) just say that you have said to your daughter to stay away and find other friends as the two girls don't seem to get along too well.