effective, especially in the lower grades.
But I did have an acquaintance who worked for KIPP charter schools in New Orleand who started teaching groups of 50 students because the school couldn’t retain a math teacher. (He was a social studies teacher but said he could take the other kids into his class and teach both subjects—totally crazy imo.) Supposedly he did a good job with it, but I’m pretty skeptical that that would be a good model to implement on a large scale.
I think a lot of good teachers would feel like they couldn’t deliver high quality instruction (personalized feedback, etc.) to so many students at one time. Maybe for some subjects it would work better than others.