I think what you are experiencing is her new found friendships and socialization taking priority over the work. I'm reading between the lines here but from what you wrote it seems that this is new to her and perhaps last year she didn't really have any close friends or if she did they were not to the level of excitement that she is experiencing now with the older kids. We all know it isn't uncommon for GT kids to be attracted to older kids and it looks like your daughter is one of those and is focusing her attention on that. So and so who got the 79% might be one of her new close friends and she might be sabotaging herself to ensure she stays in the good graces of her friends. She doesn't want to be the best for fear that she will alienate her new friends?
My angle of approach would be to talk about it and help her make the connections that if she doesn't perform she won't be in the class with her new friends.
