Haha, this should go w/o saying, but the supplemental essay is meant to SUPPLEMENT your academic/EC achievements - of course it's not going to show those off. What they're looking for is quality writing and evidence that you have a life outside of school work. Just make it interesting and unique. I would get one of your teachers to look over it, but don't stress.
my supplement was about my obsession with pop tarts lol i had the same worry at first but my guidance counselor said its good to show a creative/human side