“There is zero evidence that reduced class size leads to better educational results.”
That’s not quite true. there are number of studies that show that children under about age 7 are helped greatly by smaller class sizes. also when class sizes get down below about eight students there is an improvement as well… The really at that point it starts to become individual tutoring rather than “classes”.
But your larger point as far as classes from about a dozen to about 35 applies, and as a matter of fact generally the larger end does better than the smaller end. the real debate there is as to exactly why the larger end does better than the smaller end. My personal guess, is that if you assume that the local school boards are making any attempt whatsoever at getting the best teachers they can, That with limited supply when you reduce the class size you are taking some of the students and giving them to the same good teacher and taking the rest of the students and giving them to a teacher who would not otherwise meet the standards of the existing teachers. while there is a pretty much endless supply of education major graduates, there is not at all an endless supply of competent teachers.
All else being equal, smaller classes should provide somewhat better results. But that is not the case. The small group with the good teacher does well while the other small groups with poorer teachers flounder. But that’s still better than the good teacher quitting and all the kids being stuck with poor teachers.