This paper contributes to the literature on the impact of early-life health on education by estimating the effect of genetic predisposition to a higher body mass index (BMI) on educational attainment and related outcomes. The identification strategy exploits the randomness in which genes one inherits from one's parents by estimating sibling fixed effects models of the polygenic score for a higher BMI. These models are estimated using rich administrative data from Denmark for over 14,000 full siblings. We find that a one-standard-deviation increase in the genetic predisposition to a higher BMI is associated with a 1.4 percentage point (4.4%) lower probability of earning a high school diploma, a 1.7 percentage point (12.3%) lower probability of a college degree, and a 1.7 percentage point (3.7%) higher probability of vocational training. An investigation into mechanisms suggests that youth with a greater genetic predisposition to a higher BMI are more likely to report being bullied, have greater school absences, and lower test scores.