How do you deal with an immature friend?

All of my friends and I, we have been together since the beginning of middle school (6th grade) and we have come a long way. You know how everyone was kinda dumb and immature in elementary school? We gradually realized that we needed to act more mature in sixth grade, and now, two years later, we are A LOT more mature than elementary schoolers. Well, all of us, except one girl in our friend circle. She hasn't seemed to have grown up yet. She'll obnoxiously say "EWWWW" if we talk about girl problems. If I say hi to one of my close guy friends, she starts singing "Sarah and Alex, sitting in a tree. K-I-S-S-I-N-G" (I'm Sarah and my friend is Alex, btw). Some of us have been through relationships as well, but she hasn't been told because of her behavior. There are other various annoying, immature things she does. All of us have come a long way since elementary school, but she seems to be stuck on that level. I still want to be friends, because I do enjoy hanging out with her, as long as she isn't acting this way. What should we do?