Background: High school dropout represents an important public health issue. This study assessed the 3 distinct dimensions of student engagement in high school and examined the relationships between the nature and course of such experiences and later dropout. Methods: We administered questionnaires to 13,330 students (44.7% boys) from 69 high schools in the province of Quebec (Canada). During 3 consecutive high school years, students reported their behavioral, emotional, and cognitive engagement to school. Information on later dropout status was obtained through official records. Results: Although many adolescents remained highly engaged in high school, one third reported changes, especially decreases in rule compliance, interest in school, and willingness to learn. Students reporting low engagement or important decrements in behavioral investment from the beginning of high school presented higher risks of later dropout. Conclusion: School-based interventions should address the multiple facets of high school experiences to help adolescents successfully complete their basic schooling. Creating a positive social-emotional learning environment promises better adolescent achievement and, in turn, will contribute to a healthier lifestyle. (Contains 1 figure and 2 tables.)