Or, rather, it has one problem that could be described a lot of different ways: it isn’t funny.
Crude is okay in comedy as long as you are laughing. You can have unlikable characters who do unlikable things, and so long as those things are enacted in a funny way or express some half twisted fantasy of what we might like to do if we were not living in the real world, the audience will normally buy in.
Here though Elizabeth Halsey wants a job that will require the least amount of effort from her so that she can have a roof over her head until such time as she can find a rich sucker to take care of her. When her efforts at teaching turn, as the genre determines they must, towards attempts to actually teach, she is still only motivated by a desire to purchase breast implants so that she can more easily snag a rich sugar daddy.
There are funny moments in Bad Teacher, most involving little snippets of wish fulfillment for teachers. I chuckled, for example, at Elizabeth’s willingness to write the the comment “this is stupid” on a paper or to tell a sycophantic student that her cookies “suck.” That’s one note, though, and it’s hammered incessantly. Never is the willingness to transgress the boundaries of teaching combined with enough insight or experience of the actual experience to form a funny scene, much less a funny arc. I’ve actually seen do-it-yourself animated Youtube videos that better express where the exasperation of teachers comes from and that would form a better foundation for a funny movie about how a teacher who didn’t care might respond to such a situation. By the latter half of the film, though, the plot has degenerated into a fairly conventional romantic (I use the term very loosely) triangle that could take place in any office or workplace environment.
All that being said, Cameron Diaz still has vast reservoirs of good will accumulated from her previous work, and while she doesn’t have enough to work with to make the movie enjoyable, her willingness to play along makes it mostly bearable. There’s a trenchant, wry movie in there somewhere in between the accounts of how the school environment numbs the brains of all involved or how standards of beauty make someone who looks like Elizabeth contemplate plastic surgery. Unfortunately, in the end, laziness rules the day. It’s easier to stick Diaz in short shorts and have her do a car wash montage, complete with gawking boys and their dads than it is to give even the slightest thought as to what makes a good or bad teacher.