I think class size depends on material, teaching method and type of student. Saying 15 - 20 is optimal is too much of a generalization.
I've had 15-17 and for some exercises/projects/discussions it's simply too small for the best learning environment. For my approach, 25-30 is optimal.
A relative of mine who had 30+ years experience as
a public school teacher said the bigger the better
had worked for her, some of her best classes had
well over 30 students.
However I think the best I ever attened myself had
8 students per instructor.
Very intensive but rewarding program.
Our school district has an excellent ratio and a low cost per student. They perform above avg compared to other state districts. Private Christian schools our kids have attended spent less than $5k per year per student and outperformed neighboring public schools that spent more than twice that amount.
More money spent does not ensure either quality or small class size. High teachers' salaries and per student spending do not ensure quality and work against small class size.
Teacher's unions are a MAJOR part of the problem rather than a part of the solution.
A couple or so years ago Bredesen cut our district by
a million $$ even though we had added a new hs that
doubled the number of high schools in the county.
Meanwhile he added four million $$ to the Memphis
district.
It made me think that the number of votes count
more than the educations of the students.
As for the private schools, the Beka program will
have students two years ahead of public schools
by two grade levels in no time and have them
doing college level work by the 7th grade.
There is no comparison really.
I was talking with some of the people I went to
school with a few years ago and they all send their
children to private schools or home schooled them.
Actually there is no public high school within a
reasonable distance.
White working class people are at the bottom of the
priority list in our modern class warfare political
system.