This paper investigates the impact of school-level social mixing on social cohesion, socio-emotional development, and educational outcomes. We exploit variation induced by a French Ministry of Education desegregation program, comparing middle schools that increased socioeconomic mixing with observationally similar schools that did not. Focusing on the schools and students with the greatest baseline potential for increased mixing, we find that exposure to a more socioeconomically diverse peer group yields socio-emotional benefits for both high- and low-SES students and strengthens social cohesion, without negatively affecting the academic outcomes of either group. These findings offer actionable insights for policies aimed at fostering social cohesion in an era of rising polarization.