While the importance of social-emotional learning for student success is well established, educators and researchers have less knowledge and agreement about which social-emotional skills are most important for students and how these skills distribute across student subgroups. Using a rich longitudinal dataset of 221,840 fourth through seventh grade students in California districts, this paper describes growth mindset gaps across student groups, and confirms, at a large scale, the predictive power of growth mindset for achievement gains, even with unusually rich controls for students' background, previous achievement, and measures of other social-emotional skills. Average annual growth in English language arts and math corresponding to differences between students with fixed and growth mindset in a same school and grade level is 0.07 and 0.05 standard deviations respectively, after adjusting for students' characteristics and previous achievement. This estimate is equivalent to 48 and 35 additional days of learning.