Despite aping its title in order to suggest quality by association, Bad Teacher has nothing in common with Bad Santa—including, alas, a genuinely nasty sense of humor. The titular monster is Elizabeth (Cameron Diaz), a retiring seventh-grade teacher forced to return to her hated profession when her wealthy fiancé learns that she's a ruthless gold-digger and breaks off their engagement. With husband prospects slim, Elizabeth decides that the way to nab a man—in particular, rich new substitute teacher Scott (Justin Timberlake)—is with gigantic breast implants.
That goal, which doubles as Diaz's own insistently self-deprecating joke about her flat chest, drives Elizabeth to do all sorts of disreputable things, like steal proceeds from a school car wash (where she reels in customers via a skimpy Daisy Dukes outfit) and compete for a salary bonus by drugging a state official (Thomas Lennon) and filching a test so that her class will perform better than all others, and especially that of ultra-cheery rival Amy (Lucy Punch). Theirs is a battle of mean versus weird, though unlike Billy Bob Thornton's anal sex-loving, child-insulting Yuletide thief, Diaz's instructor is a faux-scandalous cretin, smoking pot and boozing but never appearing wasted, talking about dirty sex but never actually behaving skankily, and mistreating kids without the legitimately cruel edge that might make such horridness hilarious.
While Amy is an unfunny sort of righteous do-gooder, she's at least bestowed with one amusingly bizarre moment in which she loses control of her smiling lips. Less fortunate is virtually everyone else, as Timberlake is reduced to a mere wishy-washy drip, and Jason Segel—as the gym teacher who both pursues Elizabeth and is obviously her perfect match—is relegated to a pleasant foil. Other funny people (Molly Shannon, John Michael Higgins, The Office's Phyllis Smith) are similarly squandered as passive bystanders to Diaz's game front-and-center turn, which screenwriters Gene Stupnitsky and Lee Eisenberg waste by positing Elizabeth not as an unrepentant scumbag without a heart, but as a phony bitch whose heart is, from the outset, destined to be exposed as softer than her callous exterior.
Rather than embracing a true no-holds-barred contempt for children, the educational system, or propriety, all easy targets which Kasdan's film fails to nail, Bad Teacher vainly strives for outrageousness via dry-humping, nude-pic blackmail scams, and desperate cracks about blacks, Jews, and "Orientals." Diaz succeeds in reasserting her sexiness through a variety of hot-for-teacher mini-skirts and unbuttoned blouses, but by the end of this foul-mouthed saga, her character isn't shocking so much as just someone looking for love in the wrong place—specifically, a comedy that's never as bad as it wants to be.