Peer quality or input quality?

"Using exogenous secondary school assignments to remove self-selection bias to schools and peers, I obtain credible estimates of (1) the effect of attending schools with higher-achieving peers, and (2) the direct effect of peer quality improvements within schools, on the same population. While students at schools with higher-achieving peers have better academic achievement, within-school increases in peer achievement improve outcomes only at high-achievement schools. Peer quality can account for about one tenth of school value-added on average, but over one-third among the top quartile of schools. The results reveal large and important differences by gender"--National Bureau of Economic Research web site.