We investigate the classwide efficacy of INSIGHTS, a universal social-emotional learning intervention for early elementary grades, on observed teacher practices and student behaviors. Twenty-two elementary schools (87% free/reduced lunch) were randomly assigned to INSIGHTS or an attention-control condition. Kindergarten and first-grade classrooms (n = 120) were observed in the fall prior to the intervention and in the spring following the intervention. Multilevel random effects regression models showed an INSIGHTS main effect on observed teacher practices of emotional support from fall to spring. This effect was magnified in first grade. First-grade INSIGHTS classrooms also had higher teacher practices of classroom organization and lower classwide off-task behaviors over the school year compared to first-grade attention-control classrooms. Kindergarten INSIGHTS classrooms improved classwide student engagement from fall to spring compared to kindergarten attention-control classrooms. Grade-level variations are discussed along with implications for future research and practice.