my best friend and i are arguing and hate eachother?

What should I do? She is telling people that I am being rude to people and has ganged up with my all time enemy. We used to be very close and together but now I think because we are about to leave school, she is starting to hate me. My friends and I tease her in a friendly way but the girl who she has told about us is the worst person for keeping secrets. My other best friend does it to her as well but she doesn't care at all and just blames it on me. So say if my other best friend makes fun of her and makes a song about her, she won't care. When I make fun of her or make a song which isn't half as bad, she just gets really rude- I don't know what's best. So me and a friend have decided that we don't really want to keep in contact with her when we move schools but I just feel guilty whenever I think about it because I have tried telling her over and over again but she doesn't listen to me. We used to be such good friends but now I think she hates me. What have I done wrong? Please help!