Children's peers influence the way they dress, what music and movies they like and their attitudes about nearly everything. So why not how well they do in school?

A new study from the Public Policy Institute of California backs up what many teachers and parents have long suspected. The study found that peers can affect students' achievement even more than which teacher they have or how big their class is.

Researchers with the nonpartisan think tank tracked individual students from year to year and class to class in San Diego over three years. Their most consistent finding was striking, even to them: Students made greater gains in the years when their peers did well.

Math scores jumped by 9 percent when an elementary student moved from a low-achieving peer group to a high-achieving one, for example.

Smaller class sizes and high teacher qualifications can help, but not as much as people believe, the researchers concluded.

Students in elementary school benefited more from smaller class sizes than students in middle and high school, while older students tended to do better when they had teachers with more education and experience.

But peers influenced student achievement more consistently than either of those factors, especially in math. In reading, peer groups mattered only in middle school.

These findings are interesting in part because of the richness of data that researchers examined in San Diego, said Julian Betts, a Public Policy Institute senior fellow and an economics professor at UC San Diego. Not only did they follow individual students, something researchers can't normally do at a state or national level, they also developed a database of teacher credentials, education and experience.

The researchers then linked each student's performance to the class and teacher.

They found students did better not only when their class did better but when the entire grade did better, which may surprise some people, Betts said. Peers set the culture and norms in a school, so in a higher-achieving grade or class it may be less "uncool" to do well, he said.

Teachers, in turn, may increase the rigor of their lessons if they have a strong class, he said.

Peers may have a bigger influence in math than reading because students can help one another more easily if they are struggling in math, Betts added.

"While teachers are of central importance in the classroom, students can and do learn from each other," he said. "We need to think of students not just as colleagues and playmates but as part-time teachers as well."

The results of the study were not surprising to Kevin Johnson, principal of Foothill High in Pleasanton. A student's motivation is critical to success in school, and peers often influence a student's level of motivation, he said.

Students often hang out with friends who have the same work ethic and spend the same amount of time on homework, Johnson said. They may turn to each other for help with assignments and have academic discussions outside class.

"Parents have a major influence," he said. "Peers have a major influence. Expectations at home and expectations at school, they all contribute to a student's success."