get as much info out of her as possible. For instance, how the bully behaves usually and the relationship between her and your girl, and how your girl usually reacts to bullying behavior. Also, by the end, thank her but ask her what she will do to prevent this to happen to your girl and other students in the future. I'm not sure if she has even thought of this seriously yet and hopefully your question will make her reflect on her own classroom management and improve it in the future.
Talk to the principle but focus on prevention of this type of incident. Don't emphasize whose fault it is. It is obvious. But, if you want your girl to stay there, you don't want the school to consider you as a trouble maker either. You just need to remind them that you are not a negligent parent and you pay attention to all the service they provide. I'm almost sure that they will apologize during the talk but the apology shouldn't be the focus.
