This study investigated the vocabulary development of children (N = 547) from linguistically and socioeconomically diverse classrooms in Germany from age 3 in preschool to age 7 in Grade 1. The results showed that for dual language learners (DLLs, n = 107) growth rates in their German majority language skills varied over classrooms. Compared to monolingual children, DLLs improved faster in classrooms with higher peer-level skills in the majority language than DLLs in classrooms with lower peer-level skills (controlling for socioeconomic status and classroom quality). DLLs showed stronger growth dynamics than monolingual children during later preschool stages. The findings highlight the role of preschool peers in DLLs' acquisition of the majority language before entering elementary school.