Yikes – neighborhood politics get really, really complicated when parents add their own layer of opinions, hurt feelings, cheerleading, coaching, and backroom negotiations.
It's great to coach your kids about what's proper, what's good manners, how to be a friend, when to back off, etc. But kids learn more from our example than from our words, and if they hear us calling foul, carrying grudges, or murmuring about what Ms. X is doing wrong, that's what we're teaching them to do, in reality.
Wouldn't it make more sense to talk to our kids about some simple truths: people are not always nice to each other. We're not always nice, either. People often do the wrong thing. We ourselves make mistakes all the time. We don't have to take everything personally. It's okay to feel sad or angry as long as we really need to, but then it's fabulous to get over it, forgive, and get on with your friendships.
People generally believe that other people hurt our feelings, but in truth, we hurt our own feelings with our thoughts and beliefs. Ideally, growing up is about learning who we really are and finding our own internal compass, our own sense value and authority. Unfortunately, many adults have been taught (and model this belief to our children) that an insulting remark or gesture marks us as unworthy or unlikeable.
Does our sense of worth really come from outside us? It's a question well worth pondering.
I'd like to add, after reading Martha's comment below, that your daughter could have been the victim of bullying, and if she felt threatened or publicly humiliated, that is quite another matter. There is sometimes a fine line between teasing and bullying. Ganging up on one child often crosses than line.