A recent editorial in Science magazine delved into some of the reasons "Why many U.S. biology teachers are ‘wishy-washy’" (link is subscription only). This clearly is a very important question, as you want children at school to be taught good science. You want the children coming through the schooling system to be able to advance onto the next level of education not carrying false ideas. We want this, so that these scholars can proceed easily through higher education and eventually be able to contribute to solving world problems with innovative and correct scientific methods.
What the researchers found in their studies about teachers teaching evolution was interesting, in that it exposes the problem with teaching incorrect ideas. It also exposes the problem that creationists want to implement in school when they say "teach the controversy" i.e. teach incorrect ideas. The study showed that the teaching of evolution problem falls squarely with the teachers, as a lot of them do not have the background in teaching evolution. It is not that they may not want to teach evolution, it is that they are either not smart enough and do not understand it, or they were never given a background in the subject to be able to teach it. So in effect what they are saying is that the only way to get teachers teaching correctly is to either get a smarter grade of teacher or teach the teachers.
I think, the second last paragraph of the editorial gives a very good summation of the results where the researcher was quoted as folows:
"Reversing direction will require breaking out of a “cycle of ignorance,” the researchers believe. “Many students lack good models for teaching evolution in public schools” because they weren’t taught the subject well in high school or college. More trainees also need better hands-on grounding in what the researchers call “the nature of scientific inquiry,” such as working in a research lab."
Now that we have some data, perhaps its time for the politicians to listen to the science and ban the teaching of creationism in any form be it home schooling or religious school. Bad ideas should not be allowed into the academic square, when it clearly has a negative impact on children.