This study examines how a wide variety of diverse friendship group attributes affect changes in indicators of school performance, social behavior, and mental health between early seventh and late eighth grade. Nine hundred and one middle school students named their friends. Independent data from these friends were used to construct friendship groups that were then characterized in terms of their mean level on measures of academic performance, social behavior, psychological closeness, and structure. Two novel conclusions emerge from the study. First, nearly all the friendship effects are domain-specific. That is, peer attributes in the school domain affect individual school performance outcomes, while peer attributes in the social behavior domain affect individual social behavior. Second, friends' grade point average (GPA) is the most powerful single friendship attribute. It has the largest total effect, the most consistent effects within its own school performance domain, and the sole reliable cross-domain impact--on an index of negative social behavior that taps into acting out and drug use. The within-domain peer effects mostly replicate past findings, though the methodological controls used here are more extensive than in past studies and so provide a stronger causal warrant.