Jennifer's Body is a teen slasher flick in the way Mean Girls is a teen comedy. The description works, but if you really want to get down to brass tacks, these movies are probably more for viewers who remember high school than for viewers who are actually in high school. They're both just so sly with their observations that most probably won't notice the post-modern vibe.
Hell -- we are informed by our protagonist, Anita "Needy" Lesnicky (Amanda Seyfried) -- is a teenage girl. Needy is the unassuming, bespectacled, and usually subservient BFF of Jennifer (Megan Fox), by far the hottest girl in their tiny high school. The two appear to have nothing in common, with Needy modestly shuffling around after Jennifer, who ruthlessly wields her sex-as-a-weapon body to the torture of every boy, and possibly girl, she meets. But, of course, as Needy explains in her voice-over, "Sandbox love never dies."
Their lopsided relationship seems to work, until Jennifer becomes infected with some kind of demonic force, and starts murdering and eating boys, forcing Needy to confront her friend's evil nature -- preexisting and otherwise. What ends up unfolding is a fun comic horror movie with excellent dialogue (which writer Diablo Cody has deftly toned down from the spastic slang of Juno) and some smart subtext about womanhood and sex and friendship and whatever else you'd expect.
This works really well because the movie's horror foundation is pretty solid. So since we're already busy being entertained with an amusing and gory narrative, it's awesome to see a Final Girl who actually gets to have sex with her boyfriend. Cody only subverts the genre after covering all her bases with a good story, and she seems to know her stuff. The only real problem is that even though there's plenty of blood, guts, and general creepiness, nothing in Jennifer's Body is really what you'd call scary. Director Karyn Kusama opts to never show Jennifer actually eviscerating anybody, instead cutting to silhouettes or reaction shots. But on the other hand, there is a ridiculously hot lesbian kiss. So, all in all, it's a worthwhile venture.