Teenage peers are perceived as being important, but there is little conclusive evidence demonstrating this. This paper uses data on the population of Norway and idiosyncratic variation in cohort composition within schools to examine the role of peer composition in ninth grade on longer-run outcomes such as IQ scores, teenage childbearing, education, and labor market outcomes. We find that outcomes are influenced by the proportion of females in the grade, and these effects differ by gender. Average age and average mother’s education of peers have little impact on teenagers but average father’s earnings of peers matters for boys.