This paper seems to confirm what a lot of people suspect about teacher certification requirements:
What Does Certification Tell Us About Teacher Effectiveness? Evidence from New York City
Thomas J. Kane, Jonah E. Rockoff, Douglas O. Staiger
NBER Working Paper No. 12155
Issued in April 2006
NBER Program(s): ED
---- Abstract -----
We use six years of data on student test performance to evaluate the effectiveness of certified, uncertified, and alternatively certified teachers in the New York City public schools. On average, the certification status of a teacher has at most small impacts on student test performance. However, among those with the same certification status, there are large and persistent differences in teacher effectiveness. This evidence suggests that classroom performance during the first two years, rather than certification status, is a more reliable indicator of a teacher's future effectiveness. We also evaluate turnover among teachers with different certification status, and the impact on student achievement of hiring teachers with predictably high turnover. Given relatively modest estimates of experience differentials, even high turnover groups (such as Teach for America participants) would have to be only slightly more effective in their first year to offset the negative effects of their high exit rates.