We leverage a field experiment across three distinct school districts to identify key pieces of a structural model of adolescent human capital production. Our focus is inspired by the contemporary psychology of education literature, which expresses learning as a function of the ratio of the time spent on learning to the time needed to learn. By capturing two crucial student-level unobservables—which we denote as academic efficiency (turning inputs into outputs) and time preference (motivation)—our field experiment lends insights into the underpinnings of adolescent skill formation and provides a novel view of how to lessen racial and gender achievement gaps. One general insight is that students who are falling behind their peers, whether correlated to race, gender, or school district, are doing so because of academic efficiency rather than time preference. We view this result, and others found in our data, as fundamental to practitioners, academics, and policymakers interested in designing strategies to provide equal opportunities to students.