The study, published last month in a new book called ““Beyond the Classroom,’’ is the work of three academics, Temple University psychologist Laurence Steinberg, Stanford sociologist Stanford Dornbusch and Bradford Brown, a psychologist at the University of Wisconsin. Together, they researched nine public high schools in California and Wisconsin. They polled students, interviewed families and observed classrooms. They compared the academic careers of students who began high school with equivalent grades but who had different sorts of friends in the years that followed. They found that the youngsters with ““more academically oriented’’ friends did better over the course of school, and that kids who hung out with more delinquent types were more likely themselves to get into trouble. They weren’t entirely sure which came first: do like-minded kids flock together, or does the flock make the kids like-minded? But, after holding a variety of factors constant, the authors conclude that ““at least by high school, the influence of friends on school performance and drug use is more substantial than the influence of parents.''

Their findings help to account for ethnic differences in students’ academic achievement. ““The conventional explanation for why black and Latino kids do poorly in school and Asian-Americans perform at the highest levels is that it’s their home environment. Our data suggests that might not be the case,’’ Steinberg says. The researchers found that Asian parents were not unusually involved in their children’s education. Indeed, black parents often were much more concerned. Still, Asian children had far more success in school.

Why? Asian students in the study found it harder to join popular crowds of ““slackers,’’ so they tended to form academically focused peer groups. Once in these crowds, Asian students studied together and pressed each other to earn A’s. The opposite was true for black and Latino students. The groups they joined tended to sneer at academic accomplishment, sometimes dismissing it as ““thinking white.''

These are powerful influences for parents and teachers to contend with. The authors offer no easy solutions. They suggest, unsurprisingly, that parents should ““know your child’s friends and steer children early in their development toward youngsters who value achievement and school.’’ Easier said than done.