A popular group issue, I know how you feel to a certain degree. Wow, your principal got involved? That's just weird, a principal, or teacher for that matter, has no rights to choose who your friends with unless its causing actual problems at school. Wow, I still can't believe that... wow, just wow.. Sorry, I'll quit freaking about that and give some actual advice.
If she'd rather be with GR then there really is nothing you can do. If she decides she wants to be friends with you and GR then that would be okay, but ignoring you and your friends when it's more convenient for her is not fair to you. If she continues to treat her like that, it might be a good idea to just ignore her and enjoy your time with your other friends. If or when she comes back to your group, do not hesitate to let her know how you felt about it. She's still your friend and you don't need to forget about her, but try not to dwell on it . She might not even realize she's doing it, I know my friend bounces back and forth between myself and the more popular group without really realizing it, though from the way your friends treating you that's probably not the case.
To sum that up, if she decides to keep bouncing back and forth or come back to you guys, let her but also let her know how you feel if she does come back. And don't dwell on it, it'll just make you feel worse so just try and enjoy your time with your other friends and leave thoughts of C out of your mind.
Though, that's also what I'd do... so maybe Lotustail's idea might be more for you.
Hopefully that helps...