* Level of education: Elementary school classes will be larger than university seminars. High school classes are generally larger than junior high.
* Type of school: Public schools will have different averages than private schools. Private schools often have smaller class sizes.
* Subject: A language class might have a different average size than a science lab class.
* Specific school and region: Individual schools and even prefectures (similar to states) can have vastly different student-teacher ratios.
While precise figures are hard to come by publicly, you can generally expect:
* Larger classes: in public elementary and junior high schools. Numbers in the 30s or even 40s are possible, though efforts are being made to reduce class sizes.
* Smaller classes: in private schools, high schools (though still potentially large), and university settings. University seminar classes might be quite small (10-20 students), while introductory lectures can be very large.
In short, without more specific details about the level and type of school, it's impossible to give a precise average Japanese class size.