This study exploits a unique policy environment and a large panel dataset to evaluate the impact of school crowding on student achievement in Wake County, NC. We also estimate the effects of two education policy initiatives that are often used to address crowding: multi-track year-round calendars and mobile classrooms. We estimate a multi-level fixed effects model to identify effects that are not confounded by other school, family, and individual characteristics. Results suggest that severely crowded schools have a negative impact on reading achievement but have no discernable impact on math achievement. Both mobile classrooms and year-round calendars are found to have a small negative impact on achievement in the absence of crowding, but a positive impact in crowded schools, though these policies are only able to partially offset the negative impact of crowding. (Contains 6 tables.)