Look at who the writers are of this movie. It will tell you the quality.

Rating: 1/10 (11 August 2011)

Gene Stupnitsky and Lee Eisenberg are the credited writers of one of the worst comedies ever made: Year One, the movie where Jack Black and Michael Cera played themselves as cave men. Here the same type of mean, non-creative, only mildly entertaining humor is at play because they wrote Bad Teacher.
Some may even say that this is a wasted opportunity with this cast. I beg to differ. You mainly have one ACTUALLY funny person in the cast and that's Jason Segel, which happens to provide the movie's only funny moments. All these other actors would never be capable of writing funny jokes in stand-up or in a movie. They are all just actors there to just read what is given in front of them, entrusting that the jokes in the movie are going to be funny. Unfortunately, not even that happens in this film.
The humor in "Bad Teacher" mainly is about shock value only. The shock value here only comes off as repulsive and definitely not funny. Where is the wit in any of the lines? What are they expecting an adult reaction to be with some of the jokes in this. Someone farted really loudly on the toilet. Reaction that didn't happen: "HAHAHA." Or it shows a kid with an erection as Cameron Diaz washes a car. Reaction that didn't happen: "OMG, did they really just show that? Hilarious."
Overall, I give this a rating of: "Bad Teacher". (Some movies just start a whole new rating.)