When many students are tardy at the secondary level, teachers must continually restart instruction or delay beginning instructional periods throughout the school day. To address the considerable amount of instructional time lost caused by high rates of tardiness, the authors investigated the results of schoolwide intervention to reduce student tardiness in a tribal middle and high school. The participating school staff implemented a schoolwide intervention that included explicit teaching of expected transition behavior, active supervision of students in common areas during transition times, and consistent consequences for tardiness. After implementation, average daily tardiness decreased substantially and lower levels were maintained over time. The authors provide implications for schoolwide intervention to improve student punctuality and recommendations for future research. (Contains 2 figures and 1 table.)