From an interview with Judith Rich Harris:
LEHRER: You emphasize the importance of teachers in shaping a child's development. How can we apply this new theory of child development to public policy?
HARRIS: I’ve put together a lot of evidence showing that children learn at home how to behave at home (that’s where parents do have power!), and they learn outside the home how to behave outside the home. So if you want to improve the way children behave in school—for instance, by making them more diligent and less disruptive in the classroom—then improving their home environment is not the way to do it. What you need is a school-based intervention. That’s where teachers have power. A talented teacher can influence a whole group of kids.
The teacher’s biggest challenge is to keep this group of kids from splitting up into two opposing factions: one pro-school and pro-learning, the other anti-school and anti-learning. When that happens, the differences between the groups widen: the pro-school group does well, but the anti-school group falls further and further behind. A classroom with 40 kids is more likely to split up into opposing groups than one with 20, which may explain why students tend to do better in smaller classes. But regardless of class size, some teachers have a knack for keeping their classrooms united. Teachers in Asian countries seem to be better at this than Americans, and I suspect this is one of the reasons why Asian kids learn more in school. No doubt there’s a difference in cultures, but maybe we could study how they do it and apply their methods here.
The tendency of kids to split up spontaneously into subgroups also explains the uneven success rate of programs that put children from disadvantaged homes into private or parochial schools. The success of these programs hinges on numbers. If a classroom contains one or two kids who come from a different background, they assimilate and take on the behaviors and attitudes of the others. But if there are five or six, they form a group of their own and retain the behaviors and attitudes they came in with.