Back in the days I went two years(5th and 6th grade) to a small school on a island with a total of about 20 kids in grades 1-6.
There were further about 20 in 9-12 as kids from neighboring island came to the same school. (They had their own 1-6 grade school)
The school was organized so that the kids in grade 1-2 where in one classroom and 3-6 in other.
Some of the subjects were taught together regardless of what grade you were in, some others were more self study with teacher moving around to help as needed.
At that high teacher to student density and every student knowing everyone else so well, it was by far the best educational environment in my school times. We moved a lot so I went to a total of six schools through upper secondary (1-12)
So my view is really that for good schooling effect you need small groups and having overall small school size helps.