The aim was to explore predictors of employment during the school year in adolescents aged 13. We report on a population-based sample of children followed-up from 5 months to this day. Parents and children answered questions on family, school, health, and work. A multinomial logistic regression was used to identify the predictors of informal and formal work. Results show that female sex and leisure time physical activity (LTPA) at age 12 were associated with an increased probability of doing informal work at age 13. Also, results indicate that 13-year-olds doing formal work for an employer or in the family business were more likely to report higher LTPA, to have used alcohol, to exhibit more delinquent behaviors, and to report lower educational aspirations at age 12. Such information must be used to devise interventions aiming at reducing the risks that school year employment may entail for education and health of adolescents.