Sandy's betrayal of Miss Brodie

I wonder why it is that Sandy betrayed Miss Brodie. I am guessing that it was one of her fantasies that got taken too far. Sandy is also the only one who enters into these fantasies. Sure, the other girls make-believe and create wild stories about Miss Brodie, but Sandy is the only character's whose imagination "comes to life." I want to know, why only Sandy? Is she meant to be the main character? That is not evident to me by the first chapter, which is when an author establishes something such as that. Let's talk about Sandy and her fantasies in class!