You are all very busy saying that this girl is a bitch. Yes, she may very well be, but what if she was only shy? What if she was only afraid of making a fool of herself if she had to talk to you alone? It's much easier to send a person a note than to talk to the person. Why would she then suggest the two of you hang out another time? Wouldn't it be much easier to just write 'Can't today sorry. Am very busy'? I (unfortunately) have experience with girls doing that. My lates crush did that
My advice: Confront her, tell her that you waited for her but didn't see her. Pretend you don't know she was avoiding you.