becoming a legal guardian of a teenager

My teenage "niece" is coming to live with us for a year next year and she will be attending school out here (we are at least 4hrs away from her parents) I was just wondering if there are any forms that I might have to fill in and lodge so that I can give her permission to attend school excursions and such.
She has decide and her parents think it is a good idea as she has been self harming a lot and the aren't able to help her