In Chorus I Sit By The Worst Singer EVER?

She transfered to my school at the beginning of this school year (August) from a school just a few blocks away. I've been in chorus longer than a lot of the girls in my class, so our instructer always puts me near the newbies. I already sit by this one girl who is ALWAYS sick and extremely overweight, but the girl on my other side is even worse. She can't sing on key for her life. And on top of that, she thinks she's really good, so she sings really loud. People have stopped to tell me that my voice has changed a lot recently (nice way of saying I sound bad), but it's not me ! You can hear her out of the whole chorus, and I can't even hear myself sing around her. What should I do? A few people are starting to realize it's not me that's singing that way though. We had a concert today and I stood somewhere else, so someone else had to sing and people started talking about it. She's really weird too.. Not to be mean, so it's not like I can just befriend her and nicely tell her. I think my instructer notices she doesn't sing very well though, but there's nothing she can do about it.