Researchers Glass & Smith surveyed more than 80 studies on class size in their "Meta-Analysis of Research on Class Size and Achievement." They determined that the major benefits from small class sizes occurred when the class had fewer than 20 students. Therefore, when "small" classes are discussed by educators, they are typically referring to classes of fewer than 20 students.
A compilation of studies conducted by the Educational Research Service determined lowering class size to fewer than 20 students has the most significant impact on learning in reading and math classes in the early grades. Students in smaller classes in the early grades are less likely to repeat a grade and earn higher average grades.
While the impact is not as large as in the early grades, there is still a positive correlation between small classes and academic success in upper grades, all the way through high school. A comprehensive study conducted by the Department of Education determined that class size was a more important factor in determining success in middle school and high school than any other variable, including school size and teacher qualifications.
Studies from four countries---the U.S., England, China, and Switzerland---presented at the annual meeting of the Educational Research Association in 2008, showed that small class sizes were beneficial not only because teachers change their style of teaching but also because students feel more engaged and in control. Students have more one-on-one time with their teachers and more opportunities to work in small groups.