"Bad Teacher" must have sounded promisingly—and gleefully—anarchic in its conception, a sort of school-set sex-reversal of 2003's "Bad Santa." The finished film is a different story, always seeming to be thisclose to the precipice of a truly outrageous black comedy that really comes out swinging and aims straight for the jugular. Disappointingly, it never reaches that goal, the screenplay by Gene Stupnitsky and Lee Eisenberg (2009's "Year One") spotty and underdeveloped in its study of a morally corrupt gold-digger who acts as simultaneous protagonist and antagonist in the goings-on. Successfully well-rounded film characters should seem to live and breathe beyond the confines of script pages. They should feel like they had a life before the movie they're in began and will have one after. This doesn't happen here. Elizabeth Halsey (Cameron Diaz) comes off as a contrivance, a woman who does unsavory things and rocks the system because it might be funny to see and not because that's who she is to her core. She's stuck in a sitcom where the breadth of the world around her extends only to the edges of the camera frame.

Elizabeth Halsey has only worked one year as a seventh grade English teacher—she has "skated by, doing the bare minimum" is how chirpy, passive-aggressive social studies teacher Amy Squirrel (Lucy Punch) describes her—but now she's about to marry into money and eagerly awaits trophy wife status. When her fiancé breaks up with her, however, Elizabeth has no choice but to begrudgingly return to her job at John Adams Middle School. Her new plan of attack in snagging a man is to save up the ten thousand dollars she needs for breast implants, and she's willing to do whatever it takes to get them. She steals money from a car wash fundraiser. She convinces parents to write her checks under the guise of being the children's after-school tutor. She plots to steal the standardized test forms so her class will get the highest scores in the school and win her a big bonus. While consistently turning down the advances of earnest gym teacher Russell Gettis (Jason Segel), Elizabeth sets her sights on wooing daft substitute Scott Delacorte (Justin Timberlake) once she learns he's the heir to a watch-making dynasty. Meanwhile, Ms. Squirrel knows all too well what her across-the-hall mate is up to, making it a personal mission to expose her misdeeds.

Director Jake Kasdan has a flair for comedy, proving just as much with 2007's overlooked gem "Walk Hard: The Dewey Cox Story," and his know-how on this front at least allows "Bad Teacher" to garner a handful of laughs. All the same, the outcome is uneven, not nearly as raucous as it should be or was no doubt intended. It's cute that Elizabeth initially shows up to class each day and pops on a movie—subject-appropriate titles like "Stand and Deliver" and "Dangerous Minds" eventually segue to "Scream"—but her drunken, hung-over histrionics never comfortably coalesce with what she is like otherwise. She is rarely ever seen drinking, and her outside life is too sparsely glimpsed—she shares an apartment with a guy she met on Craigslist (Eric Stonestreet), but is only seen in those few scenes planted on the living room couch—to become three-dimensional. Likewise, when she turns to actually teaching her class after learning about the bonus, she seems to be genuinely well-versed on books like "To Kill a Mockingbird" and "Animal Farm" rather than the neglectful slacker she is presented as the rest of the time. Does she have an underlying interest in literature? Is she far more intelligent than she lets on? What are her true aspirations beyond getting fake tits and being financially taken care of? The viewer wonders these things, but no answer is provided. Elizabeth remains a sketch when it would have been far more fascinating to learn how a relatively bright person turned into such a shallow witch.

The written page might be lacking, but one cannot fault Cameron Diaz (2010's "Knight and Day") for her game turn as Elizabeth Halsey. Leaving her inhibitions at the door, Diaz jumps into the assignment of making her character as despicable as possible. The key, though, is that she's not unpleasant to watch. Diaz is so naturally likable that it wouldn't have hurt to have taken Elizabeth even further. Yes, she does some very unethical, even criminal, things, but there is a tinge of a heart hiding beneath her steely exterior. By the end, director Jake Kasdan wants audiences to care about her, but there's simply not enough insight into her beyond the surface to quite achieve this. It doesn't help that, yeah, she kinda-sorta ought to be in jail. As possible suitors Russell and Scott, Jason Segel (2010's "Gulliver's Travels") is an endearing everyman type who does as much as he can with limited resources, and Justin Timberlake (2010's "The Social Network") awkwardly stumbles to get a grip on his character. Diaz appears to be attracted to him from the moment she lays eyes on him, but he is such a flighty dunce that it's difficult to believe she'd be interested in anything but his money. As it turns out, the brightest spots in the ensemble come in the form of two outstanding supporting performances. Lucy Punch (2010's "Dinner for Schmucks") is a zany riot as Elizabeth's main adversary Amy Squirrel, a master class of comedically potent talent, and Phyllis Smith (TV's "The Office") hits the mark on her every priceless line delivery as Elizabeth's meek confidante and co-worker Lynn Davies. With out-there lines like, "I love my summers...fresh corn," enough to bring down the house, Smith is a delight every time she appears.

"Bad Teacher" alternately isn't mean enough or soft enough. With a few tweaks of the language and the deletion of a couple raunchy insert shots (one involving a middle school boy's "excitement" at seeing a scantily-clad Elizabeth, the other the outcome of a bizarre dry-humping session between Elizabeth and Scott), it could almost have been rated PG-13. It's thankfully not and sticks to its guns, but there is still the suspicion that director Jake Kasdan wasn't allowed to go as far into the realms of bad taste as he might have wanted. On the other end of the spectrum, the turnaround at the end that hints Elizabeth has turned a new leaf is too abrupt to feel like a cohesive arc. Watching the movie, it frequently feels like scenes are missing, the narrative jumping from scene to scene without necessary bridges that could have given weight to the anti-heroine and her tribulations. "Bad Teacher" is quick-paced and diverting, to be sure, and there are at least two sterling uses of song (Tommy Tutone's "867-5309/Jenny" and Shawn Mullins' "Lullaby"), but when the end arrives at the 89-minute mark the immediate reaction is that it was too slight by a half and a lot safer than expected. It's not difficult to see that the project started as something greater than it ultimately finished as.