Do teacher credentials matter to student achievement?

I've read conflicting articles on this issue with most saying "not much." This abstract contends that they do, especially for math teaching - http://papers.nber.org/papers/W12828. This article is less positive, contending that licensure typically correlates to student achievement, especially in math, only when the "bar" is being set high, which most licensure tests do not do - http://www.uark.edu/ua/der/EWPA/Research/Teacher_Quality/1798.html.