Reported by the NEA (National Education Association) with regard to a study prepared by Nichols, Glass, and Berliner, the NEA states the following:
Using several analyses of National Assessment of Educational Progress (NAEP) test data from 25 states, a link between pressures associated with high-stakes testing and student achievement could not be established. The results of this research suggest that increases in testing pressure are related to increased retention in grade and drop-out rates.
The study, High-Stakes Testing and Student Achievement: Problems for the No Child Left Behind Act,” was conducted by Sharon L. Nichols, Gene V. Glass, and David C. Berliner of the Education Policy Studies Laboratory at the Arizona State University College of Education.
It is certainly worth reading the executive summary if not the full report from EPSU.