Talk to daughter, principal, guidance counselor, nurse--if daughter is asking nurse to let her call home to go home early. Depending on what type of bullying it was, I'd go to the school board or the police or CPS or a lawyer depending on what took place and their response to things on their end. I would take her to counseling and try to help her with new coping methods, if she had hidden the bullying from me. We had good discussions so that type of thing wasn't really hidden from me by her.
Growing up bullied at home and school, and not having help on either front kinda left me feeling like a social justice warrior, lol. So I've been careful to pay attention to the signs.