We conduct a field experiment across three diverse school districts to structurally identify student motivation and estimate productivity parameters in a model of adolescent human capital development. By observing exogenous variation in 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 have difficulties converting study time into completed assignments and proficiency improvements. The study also shows that 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 provide a suite of policies to reduce racial performance gaps and suggest that school quality differences account for a substantial share of the racial differences in test scores.