After I'd been teaching at a school in Watts for a while, I learned that my predecessor had once been overpowered and left tied to his chair by an angry class. This struck me as notable because
A. Despite its location, this was not a rough school
B. My predecessor had been tied to a chair.
When I pressed other faculty members for more details, they explained (rather nonchalantly for my taste), "he was a really bad teacher."
Obviously, most jobs are easier and more pleasant if you're good at them, but this is particularly true in education. Teachers face constant, immediate and often intense feedback from students, something that is greatly intensified when you go to disadvantaged schools in the inner-city or poor rural areas like the Mississippi Delta (where I also taught).
Students get angry at bad instruction and they take advantage of bad classroom management. When you add the amplification that comes with the complex social dynamics of kids and adolescents, teaching can be a truly miserable job if you can't get the hang of doing it right.
This is a large part of the reason why so many new teachers leave the profession. Even after having invested years of study and tens of thousands of dollars, they walk away with a degree that's good for little else because, for them, the job actually is that terrible. By contrast, for those who are good at it, who can explain ideas clearly and establish a rapport with kids and keep a class focused and on task, teaching can be a most enjoyable and satisfying job.
You don't have to be a statistician to see the potential selection effect here. It should certainly be addressed when discussing the impact of bad teachers or proposing incentive pay/dismissal plans for improving education.
It should be addressed but it usually isn't.