"Approximately 15.7 percent of teachers leave their posts every year, and 40 percent of teachers who pursue undergraduate degrees in teaching never even enter the classroom at all. With teacher effectiveness a top priority of the education reform movement, the question remains: Why are all these teachers leaving—or not even entering the classroom in the first place?"
http://www.theatlantic.com/education/archive/2013/10/why-do-teachers-quit/280699/