Purpose: During the teenage years, many adolescents start drinking alcohol, and binge drinking is prevalent. We investigated the relationship between alcohol intake and academic performance. Methods: We conducted a longitudinal cohort study by combining data from the Danish National Youth Study on 65,233 high school students aged 15–20 years, with information on dropout and grade point average. We assessed associations between alcohol intake and academic performance using multilevel Poisson regression and linear regression, accounting for dependency between students from the same school and class. Results: The average alcohol intake was 10 drinks per week, and 43.6% engaged in binge drinking 3+ times per month. During follow-up, 9.8% of the boys and 6.7% of the girls dropped out. The incidence rate ratio was higher in never drinkers, frequent binge drinkers, and those with a high weekly alcohol intake as compared to those with a low intake. For example, the incidence rate ratio was 1.47 (95% confidence level: 1.24, 1.76) in girls who drank 21–27 drinks per week and 1.29 (95% confidence level: 1.13, 1.48) in girls who never drank as compared to those who drank <7 drinks per week. Alcohol associated with a lower grade point average over the entire span of intake in a dose-dependent manner, and similarly so in boys and girls. Findings were consistent in strata of socioeconomy and individual academic ambition. Discussion: Alcohol intake has implications for academic performance and poses a threat for the prospects of the individual as well as society. Policies and interventions aimed at lowering the intake among high school students are warranted.