I agree 10000% with bradian. Ignore it completely. Dont act angry, sad, or happy. Just be like stone faced. It is really hard to do this in the begining...but it gets your point across more than anything. Good luck!
I'm sorry this is happening to you. That happened to people I knew in high school too.
This may sound lame, but I swear it works! Ignore the F outta them. For like a month. It's going to suck for both you and your friends. But they will stop when they realize that the name creates no reaction out of you anymore and then it's not amusing for them anymore, and they will drop it.
Hugs-Bradian
Are they calling you it just to make you angry? I think that sometimes people will just keep going and going with something if they think it's funny, and it becomes even better for them if you get mad and start acting out because of it. It's really rude, and if I were you I would be freaking out, too :( I think what you should do is talk to each friend seperately, and tell them each that they need to stop or you won't talk to them anymore. If they keep going, then just stop. A close friend would have stopped the instant you asked them to.