The Tennesse STAR program was an experiment on the effects of class size. It has been called "one of the most important educational investigations ever carried out." The subjects were 6385 students who were beginning kindergarten. Each student was assigned to one of three treatments: regular class (22 to 25 students) with one teacher, regular class with a teacher and a full-time teacher's aide, and a small class (13 to 17 students). These treatments are levels of a single factor: the type of class. The students stayed in the same type of class for four years, then returned to regular classes. In later years, students from the small classes had higher scores on standard tests, were less likely to fail a grade, had better high school grades, and so on. The benefits of small classes were greatest for minority students.
From Introduction To the Practice of Statistics by Moore at al. The book point those interested to this webpage. It has the specifics:
In the paper "Carry-over Effects of Small Classes" J.D Finn, B.D. Fulton, J.B. Zaharias, and B.A. Nye reported that "The results of Project STAR show clearly that average pupil performance in the primary years can be increased (with reduced class size) by approximately one fourth to one third of a standard deviation without the introduction of new materials or curricula and without retraining the teachers." They also stated that in contrast to other education reforms that focus on specific subject areas and generally require some reorganization of course content, teaching strategies, and/or class scheduling "the effects of reduced-size classes were found on every achievement measure administered in Project STAR... To realize performance gains as extensive as this through any combination of student grouping, individualized instruction, or tutoring would be both difficult and expensive, if it were even possible to implement or maintain such an approach"
"...What our age needs is education. And so this is what happened: God chose a man who also needed to be educated, and educated him privatissime, so that he might be able to teach others from his own experience." -- The Journals, Soren Kierkegaard