It's time to learn: understanding the differences in returns to instruction time

As hours per day are inherently a limited resource, increasing daily instruction time reduces the amount of time pupils can dedicate to other activities outside school. We study how the effect of longer school days on achievement varies across students and schools. We exploit a large-scale reform of school schedules that substantially increased daily instruction time in Chilean primary schools. We show that the average effect of one additional year of exposure to the longer school day on reading and on mathematics test scores at the end of grade 4 masks substantial heterogeneity. Students from disadvantaged backgrounds benefit more from longer schedules, indicating that returns to time spent at school are larger the scarcer the learning opportunities available at home. Added instruction time yields higher gains in charter than in public schools, suggesting that more autonomy on administrative and pedagogical decisions may increase the effectiveness of other school inputs.