It certainly can be true that research in agreement with an expected outcome may rise to the top in school districts, or even statewide depending on what group is buying (research is not free).
The Doc, however, has awareness of these things within the scientific community (hence his "peer review" comments elsewhere), and it would be interesting to see what research he might bring to light without the burden of a desired outcome. As would be your assessment of same.
In our state the younger grades are where the emphasis is for smaller classrooms, as you note. We've not heard much about the later grades with regard to class size, what we hear in those grades is the maniacal approach to testing, and its effect on teachers, teaching "to the test", with the suggestion that is contrary to a teacher's desire.
Times have changed, but I don't think people have changed so much to validate "data-driven" science as an overwhelming factor. It has its usefulness, but does it stifle creativity?