How can I act like I'm better off without them?

Well, if I were you, I would just tell that them that I don't want to be friends with them any longer. Say that you've grown apart and that you'd like to handle this maturely. Say you want to be neutral, not friends or enemies. Just three people attending the same school that are respectful to each other, like you are with everyone else. Playing the maturity card will show them that you're over fighting and that you've grown up while they have not. (hopefully, anyway.) I really hope I could help. This situation is all too familiar to me, I know the caliber of it's suckiness. Best of luck to you!