Silver Screen: Bad Teacher ***

It's hard not to think of the exceptional, totally underrated Billy Bob Thornton vehicle Bad Santa when watching Bad Teacher, and the title is far from the only similarity. Bad Santa is about a substance-abusing, sexually indiscriminate con artist exploiting a job working with children for his own gain. That exact one-line descriptor fits Bad Teacher, a dark comedy similarly enamored with crudeness and spite, and star Cameron Diaz's character Elizabeth Halsey might as well be the slightly more accomplished little sister to Thornton's wonderfully amoral mall Santa. Same premise, different setting, copyright technically uninfringed upon. While Bad Teacher lacks the commitment to depravity and misanthropy (as well as the subsequent black-humored uplifting message) of its Bad predecessor, it still works, if only as a joke-delivery system and a showcase for the supporting players.

Diaz's Elizabeth is exactly the kind of teacher antiunion activists think of when they attack the tenure system. For her, teaching junior high is the easiest job she can maintain while still supporting herself, thanks largely to the apathy of the students and the gullibility of her fellow staffmembers. Elizabeth shows up hungover, gets high, and spends her class time showing movies in which dedicated educators inspire their charges. (She starts with Stand and Deliver and Lean on Me but gradually lapses into tangentially school-related fare like Scream.)

She spots her ticket out of the business when she meets Scott (Justin Timberlake), the newest instructor at John Adams Middle School, a naï ve but well-intentioned scion to a wealthy family. He's recently been dumped, and to casual conwoman Elizabeth he's an easy mark. The only weapon she needs to land her prey is a new set of boobs, but with her poor credit, she can't get the plastic surgeon to approve the procedure. A slew of early schemes help her net a little cash, but she finds the prospect of a big payout in the $5,700 bonus check awarded to the teacher whose class scores the highest on their state achievement tests.

That's where Bad Teacher's plot should theoretically kick in, and it does, kinda. Suddenly motivated, Elizabeth has the kids hitting the books and taking quizzes with violent repercussions for wrong answers. With the methodology of her madness applied to the appropriate goal, she becomes a kind of evil force for good. Sort of, anyway, but any time the story begins to gain momentum the movie wanders off on a tangent.

Much of the second half of the movie is dedicated to her relationship with two supporting players, Russell (Jason Segel), an amiable gym teacher persistently seeking Elizabeth's affection, and Amy Squirrel (Lucy Punch), a unbalanced, chipper teacher who also has eyes for Scott. The striking Punch, who was so funny in her limited screen time in the otherwise flat Dinner for Schmucks, is a fun antagonist for our antiheroine, and Segel gets a lot of laughs despite having the film's most underwritten character.

Bad Teacher's primary flaw is its flimsiness. It feels as hastily planned and aimless as one of Elizabeth's lessons. All the elements are in place here for a movie— a story and a conflict and solid characters, not to mention good actors in the roles— but none of it adds up to much. There are several big laughs and a handful of good scenes, most of them Elizabeth's interactions with her confused young charges, but talented director Jake Kasdan (The Zero Effect, Freaks and Geeks), working from a script by The Office writers Gene Stupnitsky and Lee Eisenberg, can't seem to tell when the movie is going somewhere and when it's spinning its wheels.

Diaz does nice work, simultaneously sexed up and deglammed. In fact, she's at her best when she's being bad, as John Cusack's soul-draining girlfriend in Being John Malkovich and especially as the murderous bride in Peter Berg's amazing dark comedy Very Bad Things. Her individual interactions with the students hint that those relationships could be deeper and more interesting, but the movie never lets her sink too low or find moments of real redemption.