I would HIGHLY encourage you to meet with the school principal. Most schools have an anti-bullying program in place. I always hesitated, thinking that by bringing it to the school's attention, I was only going to make the problem bigger.
Earlier this year, my 14-year-old son was being antagonized by a girl (she wanted to be his girlfriend and that was fine for awhile but then she got posessive, nasty, disrespectful.....I could go on an on) Anyway, I finally spoke with the assitant princiapl and he found a creative way to let on that he was aware of her behavior without it ever being revealed that we had spoken to them. I learned that assistant principals are very creative and crafty people! Trust that they CAN help improve the situation without making it worse.
ANd, my personal experience, again with this same girl, was that approaching the mother served no purpose. As much as I liked and resected the mother and her parenting style, she was in absolute denial about her daughter's behavior, even though much of it was in print (text messages and emails) I came to the conclusion that even when the mama-bear knows her cub is wrong, she is going to protect that cub (At least in front of you....what happens when they are alone to discuss it....I'm sure very different) Anyway, when I had exhausted the mom to mom approach, I went to the school and wondered why I hadn't done that first!
As for they why, typically, the bully has her own issues and is venting in an inappropriate way; whether to get attention, make herself feel empowered where she otherwise feels powerless, as a mechanism to deal with her jealousy, or to simply make herself feel better, it is unacceptable. And there are far too many avenues availaible for kids to harass and tease now (internet, texting, in addition to the old classroom and playground forums) so take it on early and aggressively.
If you read books at all, I highly recommend a book I read a few months ago titled, "Please Stop Laughing At Me" by Jodie Blanco. It is outstanding and really gives insight into the mind of the victim as a child, and as an adult after she got through it.
I hope this helps,
D.