This paper investigates the determinants and malleability of noncognitive skills. Using data on boys from the National Education Longitudinal Survey, I focus on youth behavior in the classroom as a measure of noncognitive skills. I find that student behavior during adolescence is persistent. The variation in behavior can be attributed to unobserved individual heterogeneity. Family and school characteristics, as well as the incentives for good behavior provided at home and in school, are important determinants of behavior. Neither the cross-sectional variation in behavior nor the variation over time in behavior can, however, be attributed to these covariates. (Contains 7 tables and 18 footnotes.)