In this preregistered within-teacher randomized controlled trial (n = 84), we tested the effects of grouping English learners (ELs) in homogeneous groups (all ELs) versus heterogeneous groups (ELs and non-ELs) on language, reading comprehension, and argumentative writing. Findings indicated no significant main effects of grouping. However, preregistered moderation analyses indicated that heterogeneous groups benefited students with higher English language skills (Hedges' g = 0.27-0.59 or 0.75-1.93 grade equivalents), whereas homogeneous groups benefited students with lower English skills (g = 0.31-0.58 or 1.00-1.55 grade equivalents). Instructional observations indicated that teachers provided more specialized strategies for ELs in homogeneous groups and more authentic questions for students in heterogeneous groups. Findings question the default use of homogeneous grouping and support considering English proficiency when making instructional and policy decisions for EL instruction.