Have you told her at all how you feel? Maybe not this very angry version but a version that would tell her how her behavior is harming you? And what effect it has on you?

Also, can you just back off of contact with her and set some boundaries for yourself? Like of she starts being negative or talking about her teacher you could change the subject or say you have to leave. Eventually she'll get the idea that you won't sit around for negativity or for teacher talk.

I'm for trying to salvage friendships, if possible. But if someone is too toxic and you need distance, then that's it.