The sophomore jinx hits in Diablo Cody’s second movie, proving that just because a writer has a checkered past, wears ditzy clothes, and has produced one outstanding screenplay (“Juno”), it doesn’t mean that her follow-up is destined to be equally great. “Jennifer’s Body” proves a disappointment for a second woman, too—director Karyn Kusama. Quality-wise, it’s closer to her last movie “Aeon Flux” than to her earlier picture, “Girlfight.”

The high-school horror comedy stars Megan Fox as the reigning princess of the campus in the unfortunately-named Devil’s Kettle, whose tendency to tear classmates to pieces figuratively takes a literal turn when she becomes a succubus as a result of a run-in with a sleazy rocker named Nikolai (Adam Brody), who makes the mistake of taking her for a virgin and sacrificing her to the powers of darkness in return for fame and fortune. From then on Jennifer seduces her male classmates and devours their flesh and blood. Her BFF Needy (Amanda Seyfried), whom we see kid Jennifer using in gauzy flashbacks, tries to stop the carnage, especially after she fears that her own sweet boyfriend Chip (Johnny Simmons) is in Jennifer’s sights.

Obviously Cody was aiming to twist the run-of-the-mill teen horror template a hundred and eighty degrees by making guys the victims rather than the girls while giving some subtext about female empowerment to the genre. (That’s certainly behind the lesbian motif that runs through the picture.) And one has to admit that she has dropped some sharp, knowing lines, often with pop culture references and current slang, into the script; on occasion she constructs an entire scene that cleverly riffs on the conventions of its models: a scene at a deserted swimming pool is a brilliant spoof of death-scene inanity, and all the material showcasing Brody—who’s great at offhanded, businesslike malevolence and has some great dialogue in the sacrifice sequence—is shot through with real satiric wit.

But what’s surprising is how ordinary much of “Jennifer’s Body” is. Though the first seduction, involving chunky football player Josh Emerson, is given a amusingly Disneyesque touch involving woodland animals, the second, where the victim is a poetic sort played by Kyle Gallner (the poor kid who only recently suffered from a haunting in Connecticut and now must endure being ripped to shreds), could have been lifted from scads of the movies this one is supposedly sending up. Worse, the girl-on-girl action and banter of the final reel is pretty lame; Joss Whedon carried this sort of thing off far more successful in the old “Buffy” TV series, and the fact that Kusama opts more for languorous mood than spitfire excitement tamps down the effect quite a bit.

Of course, there are those out there who will rejoice simply in seeing more of Fox—you know who you are, fellows—and she’s certainly creditable as a bitch; she must also feel good about being liberated from interacting with robots and automobiles. But this is by no means a breakthrough role for her. Seyfried is also fine, though she’s impeded by the big nerdy glasses she has to wear, and a sequence in which she has to run about wearing a hideous formal dress is far from flattering. She also has to deal with the wraparound flashback structure Cody chose—not wisely—to give the script, which results in a lot of dull expository monologues for her to deliver; the only saving grace is that allows for a good final twist that runs through the credit crawls at the end (in other words, this is one of those pictures you shouldn’t leave too fast).

Among the others, Brody scores highest, but young Simmons is a likable presence, and the seemingly ubiquitous J.K. Simmons (Juno’s dad, among his many roles) a nice spin to the few lines Cody’s devised for his hook-handed teacher, a guy who tries unsuccessfully to be “with it.” Amy Sedaris, though, is wasted as Needy’s mother. On the technical side, the movie is certainly a cut above such fare, with M. David Mullen’s moody cinematography a particular plus. But the score by Theodore Shapiro and Steven Barton is rote stuff, particular when it supplies the abrupt fortissimos to the cheap “gotcha” moments Kusama periodically resorts to, and Plummy Tucker’s editing could have been more rigorous at some points.

You have to give Cody and her cohorts credit for trying to pump some new blood into a stale genre, and there are moments of inspired cleverness in “Jennifer’s Body.” Bur they’re not frequent or sustained enough to make this the new “Heathers.”