A variety of universal school-based social and emotional learning (SEL) programs have been designed in the past decades to help children improve social-emotional and academic skills. Evidence on the effectiveness of SEL programs has been mixed in the literature. Using data from a longitudinal follow-up study of children (n = 414) originally enrolled in a clustered randomized controlled trial (RCT) when they were in Head Start, we examined whether universal SEL services in third grade were associated with the development of children from disadvantaged families. We took advantage of pairwise matching in the RCT design to compare children who had similar family background and preschool experiences but received different doses of SEL services in third grade. The results showed that the frequent (i.e., weekly to daily) exposure to SEL opportunities was associated with favorable social-emotional and academic development in third grade, including increased social skills, student-teacher relationship, and academic skills, as well as reduced impulsiveness. (author abstract)