So why exactly *did* she stay?

Because she's afraid of the real world outside. As a young girl she was sent to the girls' boarding school and this experience has a life-long impact on her psychologically. She still won't put more than five objects on the nightstand. She has serious problems communicating with anyone outside school. She has to rehearse the lines numerous times when going outside to buy food, and is extremely uncomfortable when people on the street look at her. When a man tries to help her with the money, she runs away like crazy. It's like, prisoners always wanna get out, but they gradually get used to the environment and when some are finally released they find it impossible to adjust to the real life and they commit crimes again to be put back in the prison where they feel "safe".

On one hand, she wants to leave the school so much, and on the other, she's too afraid. She is like her swimming team that is always practising but never competes, because she is never "ready" for the real thing. So she makes up these stories. It's like "inception", she builds up a perfect dream for herself and never wants to wake up to the reality. And it was all going fine until Fiamma showed up. I don't think this is a lesbian love story and I don't view Miss G as a lesbian. She clearly does't "love" Fiamma in a normal way. She's OBSESSED with Fiamma cause she wants to be her. Fiamma is exactly the way Miss G imagined herself to be: fearless, wild, adventurous. She killed Fiamma not because she doesn't love her back but because Fiamma wants to tell people the truth about her.

I think the film is about accepting the truth and facing the real world. Di didn't want to face the truth at first too (She saw Miss G kissing and touching the passed-out Fiamma but rufused to believe it and instead accused Fiamma of being a whore who seduced Miss G), but eventually she came to accept the person she idolize so much is in fact a liar psycho paedo murderer. She told others the truth, confronted Miss G and left the school to go on a real journey, the one Miss G has been dreaming about all her life.