Instructional time is a fundamental educational input, yet we have little causal evidence about the effect of longer school days on student achievement. This paper uses a sharp regression discontinuity design to estimate the effects of lengthening the school day for low-performing schools in Florida by exploiting an administrative cutoff for…

Increasing the number of hours students spend in school each year, on the assumption that this will improve student achievement, has become a widespread trend. However, the analysis reported here suggests that this trend can be misguided: the time students spend in the classroom is not always positively related to their academic achievement.…