[Ammi]

..I haven't read through all of the replies..this is a hard one....
I don't know how she would feel..about you both taking to parents together...and then to the school..but that isn't an easy thing to do..but this can't just be ignored..and you can't and shouldn't try to deal with it on your own..and violence is never the answer..it only leads to more violence often
...as cliche as this is..I think you should try to work out a way..that you can both tell a parent together..and then it's going to be..one step at a time..you can't think about any if's or but's or consequences..otherwise there's very little chance of a resolve..and problems like this can only be tackled a little bit at a time..she's reached out to you..and given you her trust..so you can't ignore that..but you shouldn't try to deal with it either..not alone..and not with violence or threats..because if that all went wrong..it could be worse for her as well..and you would feel responsible..
....try to persuade her to tell your parents...together