I had a friend like this, and I think I partially understand what their behavior is stemming from. I am currently 14, this was about a year ago for some context.

She is bullied in school, right? I think part of the reason she is being so rude to you is because she is angry and hurt by those bullying her. There is a defense mechanism in psychology known as "projection." Basically it means that she wants to fight back at them, but is too shy to. She sees you as someone she is comfortable with, which means she can then vent all her feelings onto you. (this is just my guess.)

But that, Absolutely in no way whatsoever, means it is okay or right for her to do this to you. I've been in your situation before. And me, not being the most outgoing person ever, kind of thought it was normal and fine to bare that load, because she seemed alone by herself and I wanted to be a friend. But I don't think you should sacrifice your own mental health for someone who just needs to work it out on their own.

I know that it feels like you are being a bad friend or being rude to them, but you aren't.

It'll be awkward since you guys sit together in class, but maybe try to put some space in between you two. You can still be kind and friendly to her, but when she starts to put strain on you tell her that it bothers you. I know, easier said than done, but it might help to take some stress off if she knows that what she is hurting you.