Using individual-level data from the Add Health surveys, we leverage idiosyncratic variation in gender composition across cohorts within the same school to examine whether being exposed to a higher share of female peers affects mental health and school satisfaction. We find that being exposed to a higher proportion of female peers, despite only improving school satisfaction for boys, improves mental health for both boys and girls. The benefits are greater among boys of low socioeconomic backgrounds, who would otherwise be more likely to be exposed to violent and disruptive peers. We find suggestive evidence that the mechanisms driving our findings are consistent with stronger school friendships for boys and better self-image and grades for girls.