Nobody likes her but i dont want to be mean?!!?!?!?

well, i get along with pretty much anybody, im bubbly outgoing and a generally happy person. (not trying to sound full of myself :L) but theres just this one girl that follows me everywhere!!. She is very annoying and i got used to her but now im just sick of not being able to hang around with anyone different because they all hate her! she was put in my tutor because it was the only one they could get her in as knobody put her name down and i just feel so sorry for her! i have 1 really really close best friend (were like sisters) then like 15 other close friends but she just tags along. if it was down to my best friend she would tell her straight something like, i dont like you, now f*ck off. but i just couldnt say that to someone. and im starting to worry because were going on holiday with the school and she has added herself in our group and nobody is going to tell her to shove off, they all look at me! she only has this one friend and i actually despise her. we had to put names of who we wanted in our groups and she snatched the pencil and scribbled her name on, so now i have to go with her too. Did i also mention she signed us up to share a room with her and there is no way i can get out of that unless i say something?, i just cant, but im close to. i just dont wanna blow up infront of the girl screaming at her but everything she does is just pi*sing me off big style.she sings my name in this creepy pedo style voice like shes about to abduct me or something and i am so angry and down thinking of a way to get rid of her does anyone have any ideas before i deafen this girl on tuesday?!?!? (i sware if she does one more little thing i will explode- seriously) plus it's not one of those friends one week, enemies the next. it's been going on for about a year now and im sick of it!! thank you to everyone who answers! :D