Adolescence is an important period of scaffolding for educational attainment, which is among the strongest predictors of outcomes in multiple domains. Parents who encourage academic success and promote self-regulation may enhance their offspring’s educational attainment. However, parents with externalizing disorders present a complex constellation of risk factors, including low educational attainment and poor parenting, and are more likely to have children with high levels of disinhibition. Previous research has identified low parental education, poor parenting and adolescent impulsivity as threats to educational attainment, but has not examined risk factors for discrepancies in educational attainment among siblings of the same family. Furthermore, studies have not examined the between- and within-family mechanisms that may explain why adolescents with externalizing parents have low educational attainment. The current study addressed these gaps by testing between- and within-family predictors of educational attainment using data from a longitudinal, multigenerational study that oversampled families at risk for alcohol use disorder. The sample consisted of 555 biological siblings within 240 families. We tested whether parental externalizing predicted lower educational attainment through parents’ own lower education, parents’ differential treatment of offspring, and impulsivity. Results suggested that between families, parents with externalizing disorders had lower educational attainment and more impulsive offspring, but did not exhibit increased differential parenting. Within families, siblings with greater impulsivity had lower educational attainment, whereas receiving more preferential maternal treatment than one’s siblings predicted higher educational attainment. Low parental educational attainment mediated the relation between parental externalizing disorders and low offspring educational attainment.