True, maybe the police might be a bit hasty, as would professional help... I dunno, you're kind of trapped; in a right pickle! The moral of supporting her, vs the moral of not being abused, vs the legalities...ergh. >.< If you run now, you may look like the guilty one. This is one absurd situation you're in. I feel for you, dude. I wish I could suggest something. Is it possible you could go to a lawyer? I think you need legal advice.

In the UK, there's an organisation that essentially lets you get help from lawyers (typically those of whom are retired and just want a relaxed professional life, freely helping folk) if you have something like that in your area, don't hesitate. Just for legal advice, if anything.

The way I see it, you were defending yourself, clearly never intended to hurt her, and she obviously assaulted you; unfortunately, there's no way to prove that.

Does she have a shady history? You likely have no history of assault 'n' that, which I would think to also be in your favor. Is she on drugs? Narcotics or otherwise; might've been why she did what she did. She could also get a psych evaluation... perhaps even, if it came to it, one could be mandated by the courts. I'm no lawyer... just trying to think logically, based on bits and pieces I've picked up about the law, over the years.

Best of luck! Please keep us updated.

PS
The parents saw that the room was dark, right? So there's a witness to the fact that you couldn't have really seen what was going on? That should go in your favor, unless they just flat-out lie, which they might do, unfortunately, to protect their daughter. ¬_¬

Perhaps you could contact her parents and explain what happened, briefly, and urge them to get her some help before she does serious harm to herself or someone.