Bitchy friend

So I have this ex friend who has just recently became and ex friend before we were always really close. I have just found out that she has been spreading shit about me behind my back trying to get people to hate me since the whole year basically hates her after what she did to her ex boyfriend. She has also recently started to steal some of my old art ideas that I told her about and she made an art account like I have and started posting the pictures of the my art ideas that she has stolen and claimed they were hers. Anyway I have just found my my new art class for this semester and she is in it and I don’t know anyone else in my class what should I do ?