Using data from the National Longitudinal Survey of Youth on more than 6,500 adolescents, traditional multivariate models replicated previous findings that adolescent's work intensity was positively related to delinquency, substance use, and problem behaviors. However, these relationships disappeared when observed and unobserved heterogeneity were controlled, suggesting important differences between students who engaged in intensive work and their peers. (Contains 69 references.) (SV)