Diablo Cody must have really hated high school. In “Juno,” the screenwriter depicted the adolescent institution as a series of obstacles on the way to maturity. In her follow-up, “Jennifer’s Body,” she literally demonizes the typical high school bully as an egotistical and sexual predator. But while the script may tread in the same indie-tastic water as “Juno,” the film itself plays out like a long, subpar episode of “Buffy the Vampire Slayer.”

The bone-thin plot involves the abduction of Jennifer Check (Megan Fox) by local rock band Low Shoulders (led by a delightfully smarmy Adam Brody.) Best-friend-since-childhood Needy Lesnicky (Amanda Seyfried) knows that something’s up when Jennifer returns with a cruel new personality and a taste for blood. When the boys at her high school start mysteriously dying, it doesn’t take Needy long to connect the dots.

As a horror movie, the film falls flat. There simply aren’t any legitimate scares on the big screen, and the film’s odd comic tones only make it harder for director Karyn Kusama to create a consistent atmosphere. When viewers finally see what transpired between Jennifer and Low Shoulders, they may ask why the film is classified as a horror flick.

As a comedy, “Jennifer’s Body” fares only slightly better. It’s never a good sign when the audience can’t tell the intentional laughs from the unintentional. Cody also makes it

difficult for them to distinguish witty from inane with a whole new set of teenage slang terms, or “Juno”-isms. If a girl is hitting on a guy, she’s playing “hello titty.” When a small number of people from the community perish in a fire, the town gets a “tragedy boner.” At one point, Jennifer says: “You’re Jell-O. You’re lime green Jell-O. You just won’t admit it.” No one knows what that means.

“Jennifer’s Body” does make for a decent metaphor for high school friendships and the unsettling way old friends can change. People see this story play out all the time — good-girl-gone-bad steals boyfriend — but here, it’s taken to a supernatural extreme. Though Cody’s script attempts to represent teenage angst, it’s too campy for that, even making reference to camp horror classic “Evil Dead.” But its on-the-nose skewering of high school life provides at least a glimpse into the more conventional dramedy that could have been, had the horror elements been left out.

Fox, still more of a marketing ploy than a legitimate actress, may find a role that helps her come into her own one day — but this surely isn’t it. For a movie presumably built around her sex appeal, it seems as though she tries to dial down her feminine charms in favor of falling back on her acting skills. They don’t make for a solid crutch.

Seyfried fares better as Needy, playing the same outsider-looking-in that Ellen Paige portrayed in “Juno.” The actress, who once played the deceased Lilly Kane on “Veronica Mars,” gets to be on the opposite end of the spectrum here, giving the audience a convincingly damaged protagonist to offset the robotic Fox. The girls make out scene (an all-too-brief selling point) is handled with more sexual tension than any one of Jennifer’s killings and momentarily lights a spark in an otherwise demure film.

Kudos as well to the casting director — aside from Fox. This is one of the only high school films in recent memory where the actors actually pass for high school students. Of course, when they’re as flat as Needy’s boyfriend, Chip, (relative newcomer Johnny Simmons), viewers may wish they’d gone with age and the acting experience that comes with it.

“Jennifer’s Body” is being advertised as a scary, sexy, R-rated comedy, and viewers are not going to be happy when they realize it is neither. But, hey, if your MySpace page is currently counting down the minutes until the “Twilight” sequel opens, think of this one like Jennifer thinks of her boy toys — as an appetizer.

“Jennifer’s Body” was written by Diablo Cody and directed by Karyn Kusama.