We examine how school schedule reforms shape students’ outcomes by studying the four-day school week, in which students attend school four rather than five days per week. We use comprehensive administrative data from Colorado, where this schedule is most prevalent in the United States. The dataset spans nearly two decades of entry cohorts and more than one million unique student records, providing us with a unique opportunity to study the policy’s long-run effects. Leveraging quasi-random variation in adoption across districts in a difference-in-differences framework, we estimate how a shortened school week affects students’ educational trajectories. In contrast to much of the existing literature, we find modest improvements in standardized test scores, lower dropout rates, and higher on-time and overall graduation rates. The duration of exposure plays a key role: students with longer and more consistent exposure experience the largest gains in persistence and attainment. Improvements in teacher retention and classroom expenditures also contribute to these gains. At the same time, heterogeneous effects indicate that disadvantaged students benefit less, raising concerns about equity. Overall, the findings suggest that a shorter school week need not necessarily harm student outcomes and can improve both performance and completion when designed and implemented carefully.