Date girl B, and then when she says "I love you" make it seem like you didn't know you were actually going out with her and thought you were just playing around and pretending you were dating her because you thought it was some kind of game. Then follow that up by telling her that you'd never love her, because she's a freak and you can't imagine a single person that really would love her. Let her know that her feelings for you creep you out and you don't want to be friends anymore because you think the fact that she likes you is really disgusting. With any luck, she'll kill herself and the school will close down for a few days in memorium, wherein which you can bang girl A.