This doesn't quite succeed as a comedy-horror movie, especially since the horror isn't especially scary or suspenseful. The boarding school setting is packed with the kinds of stock characters one would expect to see in a movie set in a boarding school. The satire of cliques, status, and class (social and economic, as well as the classroom) doesn't break any new ground, and the use of fracking as a central aspect of the story comes off as clumsy and forced. Furthermore, so much of the humor is rooted in the kind of locker room humor from '80s teen movies, self-aware or not, that the movie comes off more as self-parody than satire.
Considering the talent involved, it's natural to expect Slaughterhouse Rulez to be much better than it is. But many of the characters never really get a chance to shine, or emerge out of their obvious and overdone stock character cocoons. The comedy and the horror of the genre work against each other, resulting in very little of either. Everything seems to cancel everything else out, for that matter. Fracking competes with suicidal gay teens, bizarre hazing rituals, rumors of teen orgies, unleashed monsters, the English class system, etc., to the point where it's all so much noise, and all of that gets drowned out by the constant profanity.