In this study, we examine the Spanish proficiency progress of elementary Spanish-English bilinguals in three types of language program models: a dual language bilingual education (DLBE) model with a 90/10 language allocation, a DLBE model with a 50/50 language allocation, and an English as a Second Language (ESL) model over an academic year. In the domain of speaking, individual student characteristics were most predictive of progress. For listening and reading, time had the greatest impact on their Spanish language growth. With writing, the most impactful moderator was the teacher, and this was most predictive in the DLBE 90/10 model. Implications point to the need to provide immersive writing instruction and to arrange DLBE schedules to ensure the inclusion of writing. Oftentimes in 50/50 DLBE models, the domain of writing is least covered within the partner language with English taking precedence due to testing accountability. As provisions for dual language bilingual education are increasing, this study holds important implications for this model's impact on Spanish-English bilinguals' Spanish proficiency. Future studies will examine outcomes across time among emergent bilinguals in their native language, along with the potential influences of the student, the teacher, and the demographics of the school and community.