Cameron Diaz has never been much of an actress. Most of her roles haven’t demanded much of her outside of being bubbly and enthusiastic, something she’s undeniably good at. Even so, while her dramatic roles crash and burn more often than not, there’s the occasional role that fits Diaz like a glove, one in which she comes alive and manages to surprise everyone. As Elizabeth Halsey, the titular “Bad Teacher,” Diaz couldn’t be better; a shockingly perfect fit in an unapologetically filthy, hilarious movie.

When money-grubbing Elizabeth (Diaz) gets dumped by her fiancee, she finds herself returning to the public school she despises in order to fund breast implants that will hopefully attract rich men in spades. She quickly sets her sights on impossibly rich substitute teacher Scott Delacorte (Justin Timberlake), all the while gleefully neglecting her job.

Again, Diaz couldn’t be better here. Her portrayal of Elizabeth is essentially a sewer of a person — a petty, superficial brat. Diaz gives it everything she has, and even finds a small measure of charm in a mostly irredeemable character.

As great as Diaz is, any good comedy needs a strong supporting cast to round things out, and here’s where “Bad Teacher” truly shines. Justin Timberlake’s ultra-sensitive love interest gets some huge laughs entirely because of how utterly egoless and unassertive Timberlake is here. His Scott Delacorte couldn’t be more feminine, and Timberlake completely commits to the daffiness of the character.

As a gym teacher pursuing Diaz, an underused Jason Segal steals entire scenes wholesale simply by standing in the background and sarcastically ripping on his coworkers. Lucy Punch, playing the school’s most committed teacher (not to mention Elizabeth’s competition for Scott), is also hilarious as her character slowly descends into insanity over Elizabeth’s shenanigans. Punch plays these late scenes with a deranged glint in her eyes that couldn’t be more unsettling or comical. Phyllis Smith (“The Office”) also shines in a few scenes as a teacher who befriends Elizabeth, and Eric Stonestreet is nothing like his flamboyant “Modern Family” character as Elizabeth’s gruff roommate.

What really makes “Bad Teacher” work is its shamelessness. Wisely shying away from social commentary about the public school system, “Bad Teacher” instead plays like a female-centered “Bad Santa” and is all the better for it. It’s rare that a film such as this, featuring an adult in some sort of authoritative position spouting obscenities at kids, doesn’t shift into a redemption story by the end, but there’s none of that here for Elizabeth. While she does become a mildly better teacher, even this is thanks to selfish motivations, and the film’s utter lack of morality or consequences make it that much funnier.

The most problematic area of “Bad Teacher” is easily its plot. Much of the story just comes together without any of the characters having to do too much, and the film is much more interested in letting Diaz or Segel riff and have fun than telling too complex a story. While this probably makes the film funnier, a bit more plot and character development couldn’t have hurt. This is especially the case in the third act, when Diaz makes a choice that, while the right one, isn’t the kind of decision her character would make if the film didn’t necessitate it.

Between this film and the hilarious “Bridesmaids,” it’s clear audiences are intrigued by the female-driven comedy, and “Bad Teacher” makes a pretty strong case for Cameron Diaz as a comedic lead, no matter how inconsistent she’s been in the past. It’s a gleefully filthy, hysterical film, mostly thanks to Diaz’s shamelessly awful lead character, and while it’s a far cry from greatness, “Bad Teacher” works more often than not.