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. Instead, it is effective teaching time that is most likely to have a positive impact on student achievement. As such, policies influencing how time at school is allocated can be a good way to improve educational outcomes. Included in this policy brief are answers to the questions: (1) Is increasing the amount of time children spend in the classroom an effective way of improving their academic achievement? and (2) In regard to other factors that might help improve student achievement, does the amount of effective teaching time (i.e., time devoted to purely teaching-related tasks) have a positive effect on student achievement?