ya, i agree with pachyderm. normally when girls or guys are talkin shit to me, i'll either point out their obvious idiocy, smile and nod like they're speaking some other language, mouth off back and use a bunch of big words so to confuse them, or like pachyderm suggested, treat them like you're so much better (which most of the time i am cuz the teens around here are shitheads). either that, or stare blankly then spit in their face. seriously, it's funny as hell. sarcasm is a lot of fun too!
it really depends on how sensitive the person is. i'm gonna be starting my last year of highschool and i can't fucking wait. it was bullshit and everyone seemed to know me in a way yet i wasn't particularily liked... they were mainly intimidated by me cuz they know my house as the 'biker house' cuz my dad rides a harley. in any case, i really don't give a rats ass that i wasn't popular or whatever, or wasn't top in class. my sister was made fun of pretty badly during her entire school career, and it definately scarred her. i tend to think people need a reality check in dealing with kids that say lame shit to you that make you feel bad, because it's much worse when you get out of school into the real world. but of course, because of my sister i realize that not all people can just learn to deal with it, or ignore it or defend themselves.
in your case i'd say the fat bitch has some sexual tension with you. you know how when a little boy likes a little girl and he'll do all the wrong things to try to let her know he likes her.. like pour sand down her skirt or throw rocks at her and stuff.
bah. so there's my extended response.