We conduct a field experiment across three diverse school districts to structurally identify student motivation and study productivity parameters in a model of adolescent human capital development. By observing study time, homework task completion, and test results, we can identify individual and demographic variations in motivation and study time effectiveness. Struggling students typically do not lack motivation but rather struggle to convert study time into completed assignments and proficiency improvements. The study also attending a higher-performing school is associated with both higher productivity and higher motivation relative to peers with similar observables in lower-performing schools. Counterfactual analyses estimates that school quality differences account for a substantial share of the racial differences in test scores, and considers the impact of alternative policies aimed at reducing racial performance gaps.