Bad Teacher reminds me a lot of my days as a teacher myself. No matter how much planning or preparation I did and how much promise the kids had, we would all end up angry and frustrated, and nobody learned anything. The major difference between actual teaching and this movie is Cameron Diaz, who I don't find funny at all.
She plays a horrible mess of a person waiting for her rich fiance to marry her. When he dumps her, she goes back to "teaching" middle school, which mostly involves showing DVDs and sleeping off the drunkies. She raises he ire of other teachers while pursuing a fey sub from a with a rich family.
It's called a comedy because it's supposed to be funny, but it's not. Diaz's character has no charm. She's not just a bad teacher, she's a bad person. You need a little bit of good to be a relatable character, and she's not. The other actors in the film (Justin Timberlake, Jason Siegel) do what they can, but the writing is so poor, there's not much they can do. You can also tell the filmmakers didn't know what to do either, as entire plot points seem to disappear. It also seemed the like the film was being edited right there in the theater. Too bad, as it had the makings of a good movie. I'd give this film a letter grade, but it needed to show up for class more often, so it gets an incomplete.