i agree with electric7rocker... dont tell her parents unless shes ready for them to know
im a cutter and i think my parents finding out would be the worst thing ever, and if someone had told them for me i think id be VERY angry!!!
i dont really know what you can do to help her... is there any way she can wear trousers instead of shorts? and maybe a long sleeved shirt? i dont know we have the option of that at my school but maybe its different at yours.
as for helping her get past cutting... ehh thats a tough one! personally i dont think shell stop until shes ready and all you can do is help her with the reasons why she cuts.
she obviously trusts you a lot to tell you about the cutting, so you must be a good friend. just help her through her problems the best you can and that will hopefully be enough to stop her cutting