Bullying of any kind is unacceptable in school, so you should absolutely pursue this issue with them. As for how to help your daughter, that is a tough one. Kids respond to things in ways that we parents will never fully understand. More often than not, we feel more strongly about certain things than they do and so we assume a level of hurt or damage for them that may not actually exist (not saying this is the case here, but just something to think about).
I have a serious problem with how kids treat each other and have really tried to impress on my kids what a good friend and a bad friend look like. The kids that only pay attention to you because they want something that you have (be it other friends, getting closer to other family members or a cool toy they want to play with), kids that appear to be your friend but are constantly trying to get you to be someone different than you really are, or kids that try to get you to do things that you know are wrong are all what I'd call a bad friend. I've tried to teach my kids that they deserve more than that from the people they give their friendship and loyalty to. As with most things we try to teach our kids, it doesn't always sink in, but every once in a while my kids get it.
It isn't easy being a parent and times like these make your heart break in a million pieces for your child. Do your best to help her understand that this isn't a reflection on her, but on the bullies and the kind of people they have chosen to be.