Class size studies

Smaller classes help keep students focused, engaged
Students behave differently in smaller classes, staying on task with greater frequency and interacting more with their teachers, according to an analysis of research gathered from various countries, including the U.S. Small classes work for children, but that's less because of how teachers teach than because of what students feel they can do: Get more face time with their teacher, for instance, or work in small groups with classmates.
Peter Blatchford, P., Galton, M., University of Wisconsin-Madison. (2008). Class Size Effects: New Insights Into Classroom, School, and Policy Processes.
American Educational Research Association Annual conference, New York.
http://www.aera.net/meetings/Default.aspx?menu_id=342&id=4850

Class size study

Reduced class size doesn't level the playing field
Reducing classroom size to fewer than 18 students per teacher benefits high-achieving students more than struggling ones, according to an analysis by a Northwestern University researcher of data gathered in a major study on the interplay of class size and learning. "While decreasing class size may increase achievement on average for all types of students, it does not appear to reduce the achievement gap within a class," said researcher Spyros Konstantopoulos.
Konstantopoulos, S. (2008). Do Small Classes Reduce the Achievement Gap between Low and High Achievers? Evidence from Project STAR. Elementary School Journal, 108, 4.