If you’re a teacher with a time machine, and you’re wondering whether to head back 50 years, then keep the following in mind:
- Elementary class sizes have decreased significantly, from 29 to 22 (on average).
- Secondary class sizes have increased slightly, from 27 to 29.
- Teachers are pretty unanimous that smaller class sizes are better.
- Researchers are more mixed. Some say class size makes little or no difference.
- Time travel is dangerous stuff, and FYI, some parts of the past were unhygienic and/or super racist.
Fifty-Five Million 
Do you have citations for the teacher preferences and the research on class size?
Aw, I prefer to traffic in conjecture and rumor, but if you insist…
I actually don’t have a citation for teacher preferences, other than every teacher I’ve ever spoken to, but I’ll check around for that.
On class size effects, there’s this, which says that researchers generally find positive effects from decreasing class size, but they’re looking at more significant decreases than actually happen in practice. “While the studies that found positive effects from class-size reductions have focused on efforts that cut classes down to 16 or so students, states have so far tended to reduce classes only by a few students.” (http://www.edweek.org/ew/issues/class-size/)
Also, Brookings Institute does a good literature review which paints a fairly ambiguous picture. http://www.brookings.edu/research/papers/2011/05/11-class-size-whitehurst-chingos
A picture of the power of unions to push an inordinately expensive reform agenda.
I could give a laundry list of alternative policies, but one I’ve seen recently which sounds promising (certainly needs a lllot more research) is classroom right-sizing rather than downsizing, the basics being to put more students in the best teachers’ classrooms:
Click to access 20131119-Right-Sizing-the-Classroom-Making-the-Most-of-Great-Teachers-FINAL.pdf
What about pre baby boomers? What was the average class size? The baby boomers did the same thing to the education system as they are doing to Medicare.