Amid growing interest from educators and policymakers in supporting students' social and emotional learning (SEL), this brief describes the development of key social-emotional skills within the CORE districts, a network of urban California school districts that collectively serve more than one million students. Specifically, we use CORE's unique SEL survey, administered in 2015 and 2016 to nearly 400,000 students in Grades 4-12, to document trends in four SEL constructs: growth mindset, self-efficacy, self-management, and social awareness. We find that, with the exception of growth mindset, these social-emotional skills do not increase steadily as students progress through school. Self-efficacy and social awareness in particular exhibit large declines in middle and high school. These overall patterns conceal important differences across student subgroups. Girls consistently report higher self-management and social awareness than boys, but their self-efficacy drops sharply relative to boys over time. Economically disadvantaged students report lower social-emotional skills across the board, but the gaps in self-management, growth mindset, and self-efficacy narrow in high school. White students report higher levels of social-emotional learning than African American and Latinx students; Asian students report similar levels of self-management as White students but exhibit declining self-efficacy over time. While these findings are based on self-reports and cannot be generalized beyond the six districts participating in the CORE SEL survey, the scope and scale of the data far exceed anything in the extant literature. [For the related working paper, see ED591085.]