Bad Teacher is just plain bad.
Which is unfortunate, because there's really nothing funnier on film than a teacher who's just in it for the meager paycheck. Jack Black created the definitive bad teacher with his Dewey Finn in School of Rock. Dewey (a.k.a. Ned Scheebly, Dewey's schoolteacher/ roommate whom he impersonates) is a lovable loser who lives to get stoned and rock out. He ultimately comes to life -- and his humanity is revealed -- when he ignites a love of music in his students.
Elizabeth (Cameron Diaz) doesn't have any such kindness hidden within. She's a superficial bee-atch who's out to land a rich husband and get a new set of ta-tas. Falling asleep or sneaking a swig at her desk are mildly funny at first. But, screenwriters Gene Stupnitsky and Lee Eisenberg have no other tricks up their sleeves. Seeing Elizabeth slumped at her desk for the fifth or sixth time is just tiresome.
Justin Timberlake has a few cute scenes as the substitute with a big family inheritance whom Elizabeth sets her sights on. Other generally funny performers are generally wasted in this bland flick. John Michael Higgins, Phyllis Smith (Phyllis from The Office) and Eric Stonestreet (Modern Family's Cam) try their best, but they're given too little to work with.
Even the affable Jason Segel, the school's gym teacher and Elizabeth's hapless suitor, flounders his way through the material. If you've seen the trailer, you've seen not just the funniest bits of the film. You've seen the only funny bits to be had. Skip it.