This is about education in general and not science, but I'm sure it is applicable to science teachers as well. There is a very high turnover of teachers each year, and researchers are still trying to determine why so they can solve the problem and improve the education system.
"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/
"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/