A Frenemy issue.. Need some advice, please?

I've a friend (who calls herself my "bestfriend" but I don't really believe in that word..) who's REALLY competitve against me and always says crap like "I'm gonna sooo beat you b****". She sometimes acts very mature (we're both 14,she's nearly 15) but sometimes she's like a bratty kid or else a sl**. I've invited her round tomorrow via facebook message but its been an hour and she hasnt replied (and i know she's logged on cause she keeps updating her status..).. I can't ditch her altogether because nearl all my classes are with her and we've got set seats (and of course i've been plonked beside her).. Pleasey help, I really need your advice!!