We analyze the effect of a Chilean school reform that lengthened the school day from half to full-day shifts on the likelihood that adolescents engage in risky behaviors. By increasing the number of hours spent in school, the reform curtails opportunities to engage in risky behaviors that may lead to motherhood and crime. We exploit the exogenous time and regional variations of the reform's implementation to identify the effects of adult supervision on the likelihood that adolescent girls become mothers, and on municipal juvenile crime rates. We find that access to full-day schools reduces the probability of becoming an adolescent mother among poor families and in urban areas, and that the reform reduced youth crime. (C) 2010 Elsevier B.V. All rights reserved.