If you were staffing an elementary school, where would you put your best teachers? In the early childhood grades so that students would have the best possible foundation, or in the upper grades where students take standardized tests?
At least in the NCLB era, it seems a pretty safe bet that principals have taken the "better staff in the testing grades" approach.
Using North Carolina's trove of longitudinal data, a recent Calder study takes a closer look at how staffing patterns may be playing out.
Researchers Sarah Fuller and Helen Ladd use teachers' performance on the state licensure tests as a proxy for their quality--an imperfect measure, but certainly worth exploring. They did indeed find "higher quality" teachers in the test-heavy upper elementary grades than in the lower grades.