A long-term study in the USA suggests children benefit from being taught in small classes over several years.
The study covered 13 years
Researchers tracked 5,000 children in Tennessee from kindergarten to when they left high school.
They found children who had been in small classes for four years were more likely than others to graduate from high school.
Pupils from poorer families benefited most, says the study, published in the Journal of Educational Psychology.
In January, a British research project concluded there was no evidence that children in smaller primary classes did better in maths or English.
A team at London University's Institute of Education studied thousands of pupils in their fourth, fifth and sixth years of schooling in England.
In the American study, students were followed for 13 years, in a follow-up to a class-size experiment in Tennessee in the 1980s.
Jeremy Finn and Susan Gerber of the University at Buffalo and Jayne Boyd-Zaharias of the social foundation Heros followed-up the experiment known as Project Star.
In the original experiment, kindergarten pupils were randomly assigned to one of three types of class.
They went either to a small class (13-17 pupils), a full-size class (22-26 pupils) or to a full-size class with a teacher and a full-time teaching assistant.
The study's authors concluded that four years in a small class in elementary school were associated with an 11.5% increase in high school graduation rates.
They said the effect was even greater for children from low-income families.
"Our results contradict arguments that just one year in a small class is enough to reap long-term academic benefits," said Dr Finn.
"Three or four years of small classes are needed to affect graduation rates, and three or four years have been found necessary to sustain long-term achievement gains."
He said the differences could not be explained just by the improvements in the children's educational achievement.
"Other dynamics must have been occurring as well, for example, effects on students' attitudes and motivation, students' pro or antisocial behaviour, or students' learning behaviour," he said.