Rumour spread to 150 people about me?!?

[Clemma2213]

So this fat kid at my school has a weird crush on me (but he doesnt even know me.. i mean,,, for all he knows I could be depressed, a druggie, drunk etc etc..) and one of his mates said I was going out with him to about 70 people... then all of those people told others etc etc... so now 150 (that i know of) think I am going out with this fat kid and cheating on my bf... I don't know what to do.. I sent the kid who first started it a message on facebook saying: Why did you say we were going out... cause we aint!" but im having people reject me because this kid is annoying.. he thinks that because he was bullied he can have his own way... (one of my mates go so mad at him... she ended up just walking out of the classroom and going home...) So what do i do?
I have people sending me messages saying: "OMG!!! You sl**... you're bf doesnt deserve you.. I bet hes going to break up with you once he finds out"
P.S. my bf already knows it is a rumour...
Thanks in advance