I suppose it revolves in part about how old she is, parents would probably be the best bet, she may hate them, but that doesn't mean the feeling is mutual, and, if she's quite young, they are probably the best people to decide how to progress.
School Counsellors are a second option, because they tend to be obliged to report incidents like this, which could lead to investigations of home-life, which usually ends up doing more damage to the family than good, but if my child was self harming, to my mind, the first step is dealing with that, she could despise me for it for the rest of her life, but at least she'd have a rest of her life in which to despise me.