WHAT WOULD BE VALID REASONS FOR MY GRANDMOTHER TO HAVE TEMPORARY CUSTODY FOR ME TO GO TO THAT CITIES SCHOOLS?

im 15 going to the 10th grade and i recently left my old school in january for varias reasons. then i became homeschooled for the remainder of the year. i recently decided to go to the school in my grandmothers city.i wanted to know if i needed a reason to do so or if the forms could just be filled out without a reason for temporary gaurdianship being stated in the documents. and does a trial need to be held in order to get the forms?