Children's working memory (WM) skills, which support both academic and social success, continue to improve significantly through the school years. This study leverages the first nationally representative data set with direct assessments of elementary school students' WM skills to examine whether WM grows more during the school year or summer months and whether WM growth rates differ by household income. Results demonstrate that WM skills grow more during the school-year months compared to the summer months, suggesting that school environments provide children with unique opportunities to improve and practice their WM skills. Further, lower-income children have significantly faster WM growth rates in the first 2 years of school and the intervening summer, compared to their peers from higher-income families, leading to an overall narrowing in WM disparities by household income during the early school years. However, there was no evidence that schools equalize or exacerbate differences in WM skills between children from lower-income and higher-income households.