In the United States, the StudentTeacher Achievement Ratio (STAR) project in Tennessee was initially conducted between 1985 and 1989. Some 7,000 pupils were tracked from kindergarten (age four) to third grade (age eight).
A third of the pupils were taught in small classes of between 13 and 17 per teacher, a third in "regular" classes of 22 to 25, while the remaining third attended regular classes with both a teacher and a full-time teacher's aide.
That initial study found the small-class children performed significantly better in reading and maths tests than their peers in both regular classes, and regular classes with a teacher's aide.
Follow-up work on STAR at Tennessee State University now shows a lasting impact later on in pupils' school life.
To assess how long-lasting the benefits were, the children have continued to be followed as their schooling has continued, even though all returned to regular classes after the initial four-year study period was completed at the end of the third grade.
Research director Dr Barbara Nye says the study has shown that not only were the small-class children performing better than their peers in grades four, five, six and seven, but they were still outperforming them in tests in the eighth grade, with almost 6,000 children still involved at the age of 13.
"The positive effects from involvement in a small-size class still remain pervasive five full years after students returned to regular-size classes, " says Dr Nye.
"Eighth-grade analyses consistently favoured the achievement of small-class students over regular-class students in all subjects, including reading, language, mathematics, study skills, science and social studies."
White and ethnic minority pupils of both sexes were all shown to have benefited educationally from the small-class experiment.
And the continuing good news for the proponents of smaller classes is that the advantages obtained by the small-class children in the Tennessee study show no sign of fading. But the study is by no means over yet.
Already hailed by Professor Frederick Mosteller of Harvard University as "one of the great experiments in education in United States history", the Tennessee research will be expanded to cover pupils' later attendance, discipline records, participation in class, drop-out rates, and progress into the wider world.