We evaluate the impacts of small high schools on youth risky behaviors and mental health in New York City, using a two-sample-instrumental-variable approach to address endogenous school enrollment. We find heterogeneous effects. School size, overall, has little effect. Among "students" most likely to attend small schools opened "after" an educational-achievement-oriented reform, however, diagnoses of violence-associated injuries and mental health disorders increased. Among students most likely to attend traditional small schools opened "prior" to the reform, pregnancy rates and diagnoses of mental health disorders declined. School focus is more important than school size as a determinant of student well-being outcomes.