An awful lot is made these days about the need for high-quality teachers in our schools. And I agree: we need good teachers in our schools. But I'm not sure that "good teacher" really means anything -- and the same goes for "bad teacher."
Kevin Carey wrote earlier about recent findings that value-added measures are inconsistent and identify the same teachers as very good or very bad depending on the year or test on which they're based. This means at least one of the following two things:
1.) The tests aren't very precise at measuring student growth
2.) The amount of student growth under the same teacher varies in different years
Let's focus on number two. If the same teacher can get good test scores one year and bad the next (or vice-versa) it may be the case that their level of success depends on many things besides their teaching ability. In other words, the particular kids they have, the dynamics of the class, the dynamics of the school, the teacher's personal life, the subject matter, the weather, and who knows what else may affect the quality of a teacher in any given year.
This makes sense to me. I'm good at teaching kids how to swim faster; I'm not good at teaching them how to dance. I can teach 6th graders about gravity, but I'd be an awful AP Physics teacher. It's easy to see that being a good teacher for one subject or age group does not mean that you'll be a good teacher for another subject or age group. And it makes perfect sense that somebody could be a good teacher for Class A and not for Class B -- or in School A but not in School B. I'd argue that teaching different subjects in different schools across different age groups requires very different skills (not to mention different areas of expertise).
In other words, it's likely that a teacher's success in a particular classroom is due at least as much to contextual conditions as it is to any innate ability. When declaring that a teacher is "good" or "bad" we should take these things into account.
Bottom line: finding the best fit for a teacher may be easier than simply trying to find the best teacher.