Do you mean a supplemental essay? Or a writing supplement, as in the optional “arts” supplement to show special talent? If the former, it has to be a new essay written specifically for the application. If the latter, I’m not sure why the piece has to have been graded. Do you not have copies of everything you’ve written? If you are trying to show a special talent in writing, do you have published pieces, maybe from the school literary magazine or school newspaper?