In “Bad Teacher,” a breezily crude comedy about unladylike pleasures like guzzling booze, swearing at children and being mean because, well, you can be, Cameron Diaz taps into her inner thug. It’s a beautiful thing. A performer with a gift for light comedy and a comically ductile face that can work in fascinating counterpart to her rocking hot body (as her character would say), Ms. Diaz has found her down-and-dirty element in the kind of broad comedy that threatens to get ugly and more or less succeeds on that threat. Written by Gene Stupnitsky and Lee Eisenberg, and directed by Jake Kasdan, the movie is high-concept low comedy and as pared down as a haiku: a beautiful woman, without an apparent conscience, wreacks havoc. There’s no hint of how nasty Ms. Diaz, or rather her character, Elizabeth, can be when you first see her tucked into a dreary field of other middle-school teachers, standing out like a yellow rose. With thorns. — Manohla Dargis