Background: We aimed to expand the literature on child obesity and school outcomes by examining associations between obesity and high school dropout, including the role of obesity onset and duration as well as ethnicity. Methods: Data on 5066 children obtained between 1986 and 2010 from the child cohort of the 1979 National Longitudinal Study of Youth (NLSY79) were analyzed. Group-based trajectory analysis identified obesity trajectories from 6 to 18 years. School completion information from age 14 into young adulthood was used to calculate school dropout. Chi-square and pairwise comparison tests were used to identify significant associations between obesity trajectories and school dropout. Results: Adolescents belonging to an increasing trajectory (adolescent-onset obesity) had a higher likelihood of dropping out of high school compared with those belonging to chronic, decreasing (childhood-only obesity), and nonobese trajectories. This association was particularly salient among white adolescents. Conclusions: Obesity onset during early adolescence increased risk of high school dropout. White adolescents were particularly vulnerable. Given that early adolescence is marked by significant biological and social changes, future research should seek to identify the underlying processes linking adolescent obesity and school dropout to decrease school dropout risk among this vulnerable population.