Maimonides' Rule, Israel
Due to rabbinic scholar Mainmonides' interpretation of the Torah regarding education, Israel has capped class size at 40 since 1969. This means that a grade of 80 students will have two classes of 40 each, but a grade of 81 students will have class sizes of 27, 27 and 26. The wide and random difference in class size provides a natural experiment.

Findings:

• Students in smaller classes had higher test scores in math and reading after their second and third years in small classes. Their advantage over students in large classes was greater in the third year than the second, suggesting that learning gains are cumulative over time. Percentile gains were roughly the same as the Tennessee Project STAR experiment.
• Researchers used data from third- to fifth-graders, demonstrating that small classes are beneficial beyond the earliest years.

Institute of Education Class Size Study, University of London
Researchers randomly selected 11,386 students in the United Kingdom and followed them from the age of 4 to 7, recording demographic variables along with class size and test scores. Teachers were surveyed about how they spent classroom time, and researchers systematically observed selected classrooms by noting interactions every 10 seconds.

Findings:

• Students in small classes gained up to 14 percentile points on reading tests compared to their counterparts in large classes, with low-achieving students posting the biggest gains. On math tests, students of all achievement levels consistently scored 10 percentile points higher in small classes. This effect is roughly the same as the effect found in the Tennessee Project STAR experiment.
• Students in larger classes were twice as likely to be off-task and spent more time talking to each other. Their teachers reported spending less time on instruction and providing feedback for individual students. 
• Students in small classes had almost 50 percent more one-on-one interaction with their teachers than students in large classes; they also received more immediate and positive feedback.