https://www.sciencedaily.com/releases/2019/12/191215201146.htm

The precise effect of smaller class sizes can vary between countries, academic subjects, years, and different cognitive and non-cognitive skills, with many other factors likely playing a role. These findings are reported in a paper in Research Papers in Education.

Smaller class sizes in schools are generally seen as highly desirable, especially by parents. With smaller class sizes, teachers can more easily maintain control and give more attention to each pupil. As such, many countries limit the maximum size of a class, often at around 30 pupils.