Existing research on self-management skills shows that measures of self-management predict student success. However, these conclusions are based on small samples or narrowly defined self-management measures. Using a rich longitudinal dataset of 221,840 fourth through seventh grade students, this paper describes self-management gaps across student groups, and confirms, at a large scale, the predictive power of self-management for achievement gains, even with unusually rich controls for students' background, previous achievement, and measures of other social-emotional skills. Self-management is a better predictor of student learning than are other measures of socio-emotional skills. Average growth in English language arts due to changing from a low to a high level of self-management is between 0.091 and 0.112 standard deviations, equivalent to almost 80 days of learning.