Using mixed methods, we investigated (a) the association of the extent of English learner (EL) classroom-level segregation (proportion EL) and number of EL English proficiency levels with elementary EL academic achievement, using 2 years of administrative data, and (b) school staff--reported opportunity to learn-related advantages and disadvantages in segregated versus integrated compositions, using 3 years of interviews. Findings were corroborative across methods. After accounting for student-, classroom-, and school-level covariates, we found that ELs in more segregated classrooms exhibited lower performance, on average, on state tests of English language arts, mathematics, and English proficiency, and little evidence that classroom number of EL English proficiency levels was related to achievement. School staff consistently detailed the instructional, academic, and socio-emotional opportunities to learn afforded by the diversity/heterogeneity of integrated classrooms.