Because Cameron Diaz is less afraid to look stupid, unsympathetic or even unattractive than other models-turned-actresses, she often comes across as more intelligent than she perhaps is. With her unfailingly fit torso, blonde cheerleader bangs and hip-shaking runway walk, she can play the ditzy floozie in her sleep, but she also has sparkling, curious eyes that convey real hurt, like a misunderstood Barbie doll. In brainy indie films like “Being John Malkovich” or pitch-black comedies like “Very Bad Things,” she risks letting her target audience down by playing characters just a tad bitchier or grungier than one would expect from an American sweetheart.

Unfortunately, that very boldness often leads Diaz into the most vapid projects imaginable. It’s refreshing that she has publicly acknowledged her love of gross-out humor, her “one of the guys” mentality, and in showcases like “There’s Something About Mary,” her delight at joining in the belching contest has been infectious. But since Diaz is more known for her brash confidence than for genuine talent, she needs a script as wittily subversive as she is defiant. And “Bad Teacher” is the dumbest, sloppiest film on Diaz’s resume—yes, it’s stupider than the “Charlie’s Angels” franchise.

“Bad Teacher” aims to be the sister comedy to 2003’s “Bad Santa,” another one-joke, R-rated premise about supposedly kid-friendly people hating and cursing out kids, with hints of the more lighthearted “School of Rock” thrown in. But “Bad Santa” went all the way with its indecencies, and had an actual plot framed around them. And in “School of Rock,” the joke was that Jack Black’s teacher was the child, learning to shed his ego through his students’ sweetness.

Here, the naughty doings are stale and broad (Diaz throws dodgeballs at the slow kids, Diaz calls a goody-two-shoes “hopeless,” etc.) and the students and subsidiary characters are cardboard ciphers. And director Jake Kasdan and screenwriters Gene Stupnitsky and Lee Eisenberg apparently forgot to throw in any wit or color or purpose to the story. There’s more laughs and shock value in a single Billy Bob Thornton snarl than in the entire 89-minute running time of this pseudo-scandalous misfire.

Diaz, looking appropriately haggard as a past-her-prime gold digger, splits with her sugar daddy fiancée and is thus forced back to the teacher position she despises. (It’s never explained why she can’t, for instance, take a job in fashion retail). A foul-tempered, pot-smoking boozer, she screens “Stand and Deliver,” “Dangerous Minds” and other feel-good movies for her class while sleeping off hangovers. She lies to the easily duped principal (John Michael Higgins) and parents that she’s taking a “multimedia” approach to the curriculum. Her getting away with this misbehavior is supposed to be hilarious, but it’s just laziness on the part of the screenwriters; “Bad Santa” and “School of Rock” presented wonderfully inventive, smart foils for their flawed heroes.

The only person on to Diaz is Lucy Punch, as a twitchy, creepily New Age math teacher, who comes up with groaners such as “I am the math terrorist and I have ties to Al-ge-bra!” She obnoxiously adheres to the school authorities and Diaz obnoxiously defies them, and, in the most inane dark comic subplot in history, Punch and Diaz compete for the affections of the dorky, bow tie-wearing, apparently wealthy teacher (played by, ho ho, Diaz’s real-life ex Justin Timberlake). Hellbent on raising money for a boob job to impress the fatally vanilla Timberlake (again, this is inexplicable; why doesn’t she just pine for hot bankers at the local bar?), she launches a scantily-clad car wash excursion and various extortion plots. None of this is ever more clever than, say, the Van Halen video for “Hot for Teacher.”

Punch gives her role her lip-chewing, eye-squinting all, but it’s in the service of a screenplay written in crayon. And Diaz’s character never resorts to anything truly outrageous, like Thornton having anal sex in a department store changing room. She writes “Are you fucking kidding me?” on a student’s essay, rubs poison oak on Punch’s apple and—in the funniest scenes—counsels the romantic loser kid to get over his crush (“Anyone who wears the same sweatshirt every day isn’t getting laid ‘til he’s 29.”) But the nastiness is so forced, so eager to please, that it rarely yields more than a few smiles.

Kasdan, Stupnitsky and Eisenberg do deserve some merit for not tacking on a syrupy redemption plot for their hostile protagonist. Diaz triumphs through cheating, blackmail and cruelty. And the “nice guy” in “Bad Teacher,” played, of course, by Jason Segel, is a slovenly gym teacher almost as lewd as her; she’s not exactly knocked off her feet by Prince Charming. But Segel appears in the film so intermittently that he barely registers. And without any charm given to Diaz’s character, without any reason to admire her, “Bad Teacher” has no center. It’s not a movie; it’s just a trite comic conceit, a stream of insults and improbable gags disguised as oh-so-daring irreverence.