My 12-year-old daughter is very well liked by the girls in her grade. However, she's not into sports like most of the girls, so they don't include her in their lives. This is very difficult for my daughter, and now she seems to be associating with some girls I don't approve of. I keep telling my daughter that she's a wonderful person with a heart of gold, and that she needs to quit lowering herself to these other girls' level.
One big mistake we parents make is putting down our children's friends. We don't realize that our children infer that we think they are incapable of making and choosing friends. When we do this, we drive our children right where we don't want them to go. Her sports-playing peers have rejected her just because she doesn't play sports. Here is a group that has welcomed her. Perhaps see her heart of gold because she has accepted them for who they are and not what they do.
Children at the middle level have to determine and learn what makes a peer a friend and how to get along with others. Instead of putting your daughter's new friends down, I would suggest that you ask her with genuine interest what she likes about them. Let her talk. Get to know them. Let your daughter invite one or two of them over after school for snacks, to listen to music, or for some girl talk. Not only do you get to oversee the relationships, but your daughter gets to learn about them in a protected atmosphere.
If you are accepting and open with your daughter, she will tell you if these girls really are bad. If she determines they are not true friends, she will stop associating with them on her own as long as she knows you are respecting her decisions and not going to say, "I told you so."