How to get rid of her nicely?

Okay this is a REAL toughie. Back in year 7 (three years ago) there was this girl I saw on my first day. She was really nerdy, big teeth (really big), gles the works. I didn't speak to her for the first few weeks but then i started feeling sorry for her so i made friends with her. And now she has just latched on to me and I haven't been able to get rid of her since. She always bangs on about us being best friends when I just really don't want to be her friend anymore. We are insanely different. I'm into partying, going out, drinking and having a good time. Whereas she is into reading, writing, playing imaginary games (at her age!!!) and History. We also are really different physically. She has no figure what so ever, no at all she is completley flat, in fact she told me she was a 28AA!! Since the years have progressed she obviously got braces but her teeth still stick out. She also has the worst of the worst bad breath and always have greasy hair and giant sweat patches. She also has no idea about the modern world, she doesn't know anything about celebrities and she's still so child-like. Me and three other girls that are her 'friends' are also really fed-up with her. And no matter how many times we tell her she needs to sort out her lack of hygiene or something she gets in a big strop and bursts into tears well crocodile tears. And I've tried breaking friends with her on numerous occions and she just gets all stroppy and gets loads of girls in my cl aganist me saying I'm bullying her. And then she stands all by herself not really doing anything and looking all forlorn so I feel sorry for her AGAIN and end up reuniting our friendship. I'm desperate to stop being friends with but just let it out nicely so she doesn't get all worked up like she always does so me and my friends don't look like nerds ourselves because she is totally bringing down our reputation and I know that sounds awful but she really is and she has made no effort whatsoever to make friends with any other girls in our year she just latches on to us all the time. Please help, she's driving me nuts!!! I've had to put with her for three years and that is enough!