Daughter Being Bullied

My 7th grade daughter comes home crying a lot after school because of a group of popular girls who are bullying her. They do not ever hit or punch her or do anything that is too obvious but they are tormenting her psychologically. They invite everyone to parties but make sure she never gets an invite. They call her names and make other people in her class laugh at her. They write notes about her and pass them around the class. Basically they are trying to do whatever they can to make her feel like she is worthless…stepping on her when she is down. My daughter is a little bit shy but she is a very sweet and smart kid and she does not deserve the way she is being treated. I have talked to her teacher. She says that she can see that some of the girls are not very nice to my daughter but she says that since there is nothing physical it is very hard for her to catch anything that is going on and intervene. The teacher is at least sympathetic and I feel like she is on my side in this but the principle is useless. His attitude seems to be that girls of this age are mean to each other and that’s just the way it normally is and he’s not going to get involved unless there is blood in the hallway, basically. What should I do? I do not know what to do at this point other than to get in touch with the parents of the girls who torment her myself. I have not done so yet because my wife thinks it’s something that could go badly and my daughter is begging me not to. So what should I do? I know how hard this can be on a kid and I know how seriously girls of her age need the approval of their peers.