Bullied by my friends, help?!?

there's this one group of friends, probably not my closest friends but they still are close to me. however, there's this girl in this group of friends called Jessica white and she's extremely over weight but I've never said anything to her about it because that's shocking. anyway, she uses her weight to defend herself, she's got into many fights and is so mean to a lot of people! she skits me a lot, telling me to shut up ( like shut up Meg off family guy), skits me because I'm one of the smartest in our year, for not plastering myself in make-up for school and i'm just a silly little teachers pet to her! i hate it, the other friends in our group all slag her off behind her back to me but they don't have the balls to tell her to her face so she still thinks she has an army behing her. all of this bullying has lead to me feeling very self concious about myself and becoming almost boring to the point that i have to get drunk before i go to school to make the pain go away so i can act like myself in school again. but this is only with this one group of friends which I'm in all my lessons with so i can't escape it, yet with every other group of friends I'm myself and very confident. so what should i do to stop her because it's becoming unbearable!? Please help!!