Horrible nasty housemate, please help?

These aren't your only potential friends. Why don't you try talking to some other people? Just because you live with them doesn't mean you have to be friends with them. They are still very much in clique-mentality, and this can often be reminiscent of school days but, as you've said so yourself, you shouldn't be handling stuff like this at your age. These girls probably had the best time of their lives at school and haven't quite moved on from that. That's sad. You don't want to be friends with these people as they sound immature and pathetic. Just walk away from the situation knowing you tried your best, be civil, do what you need to do around the house and leave them to it. Even better, is it a possibility to move out? Uni is supposed to be an opportunity of a lifetime as well as a means to a better education, so why let these girls drag you down? Could be another reason to get chatting to different folk, maybe? Hold your head up, hun! And don't worry... You'll probably do better in your degree than them anyway ;)