Researchers have shown that children's social-emotional growth is inextricably connected to academic learning. We developed the Social-Emotional Learning Foundations (SELF) intervention, a Grade K-1 curriculum merging social-emotional learning (SEL) and literacy instruction, to promote language supported self-regulation, specifically for primary grade children at early risk for emotional or behavioral difficulties. We report findings from a pretest-posttest cluster randomized efficacy trial with one fixed between-subjects factor to test the effects of teacher-delivered SEL instruction against those of business as usual (BAU). We recruited 163 kindergarten (K) and 141 first grade teachers from 52 schools across 11 school districts within one southeastern state. Our student sample (n=1154) consisted of 627 kindergarteners and 527 first graders identified by teachers as at risk for internalizing or externalizing emotional and behavioral problems using the "Systematic Screening for Behavioral Disorders"; 613 of these students participated in the SELF condition and 541 participated in the BAU condition. We randomly assigned schools to SELF or BAU and used a multilevel model with three levels (i.e., children, classrooms, schools) to analyze data on subscales of six (four teacher-report and two direct) assessments related to self-regulation, social-emotional learning, social-emotional vocabulary, and general behavioral functioning. We found positive main effects of SELF compared to BAU on all but one measure, with effect sizes (calculated using Hedges' "g") ranging from 0.20 to 0.65. Findings provide evidence for guiding future SEL intervention research and informing practice to improve student outcomes, particularly for children at risk for behavior problems. [This is the online first version of an article published in "Journal of School Psychology."]