Someone started a rumor that I liked someone in my class when I don't. What should I do?

Just ignore it. If you don't acknoledge that as true, they will stop sooner or later. Tell your friends that it isn't true. If they keep bothering you about it and teasing you about it, tell them to stop. Tell them that you really don't like it when they say that and it really bothers you. Your true friends will leave it aone and be there when others bring it up. After a while, people will forget all about it. Don't let it bother you. It is just a phase.