Yes I see that also, kids whose lives seem really awful to me, still want to do hearts and flowers. I'm not saying that high school kids shouldn't do depressing art, sure they should if that's what the need to express, art can be a lifesaving outlet.
My observation is more that if I were a happy go lucky kid with a pleasant comfortable life and saw that last year's entire statewinner exhibit was all grim and sad and depressing, even better if the title involved rot or decay, then you can bet that's the kind of art I would want to make for competition, especially if there was scholarship money involved. It is self-perpetuating in a way, and after looking at the entire set of slides, I actually started wondering if some of them were actually parodies of high school angst rather than the real thing.