Well, I meant that as a half joke, but I'm still surprised that I'd need to explain the "horror" in that movie..
There's nothing more horrible than ignorance, expecially on kids it can be even more ruthless. The attitude that is so well portraited in that movie is disgusting. The things that happen in the movie can and do happen in real life. Unlike with most of the horror movies you can't just switch the tv off and go "that's not real" unless you're as ignorant as the kids in the movie.
For me the Kids is a horror movie of the worst kind because it's a reflection of reality.