Using value-added models, we find that high schools impact students' self-reported socio-emotional development (SED) by enhancing social well-being and promoting hard work. Conditional on schools' test score impacts, schools that improve SED, reduce school-based arrests, and increase high-school completion, college-going, and college persistence. Schools that improve social well-being have larger effects on attendance and behavioral infractions in high school, while those that promote hard work have larger effects on GPA. Results suggest that adolescence can be a formative period for socio-emotional growth, high-school impacts on SED can be captured using self-report surveys, and SED can be fostered by schools to improve longer-run outcomes. These findings are robust to tests for plausible forms of selection.