The goal of this study was to assess the effectiveness of an intervention--Quality English and Science Teaching 2--designed to help English language learners (ELLs) and their English proficient classmates develop academic language in science, as required by the Common Core State Standards. The intervention consisted of supplementary instructional materials and professional development. Participants included 1,309 students, of whom 353 were ELLs. Sixty sections taught by 15 teachers in seven middle schools were randomized within teacher to the intervention or the control condition. Treatment effects were tested separately for academic language and science knowledge. For the whole group, results indicated posttest differences favoring the treatment group sections were statistically significant for academic language and science. For ELLs however, posttest differences favored the treatment group for academic language only. Follow-up analyses investigating implementation found that treatment fidelity was related to gains in both academic language and science knowledge for the group as a whole and for ELLs. Effect sizes suggest the intervention was promising in promoting the academic language in science of both ELLs and their English proficient classmates.