Despite a clear finding that immigrant children in bilingual education programs outperform children in English-only instruction, little is known about the underlying causes of this effect and the variability in the results. This study seeks to understand cases in which bilingual students with emerging English skills appear to experience success or rapid academic gains in English-only classrooms in the apparent absence of home language support in school. Using a sample of 2,428 Spanish-speaking bilingual students in 438 schools from the Early Childhood Longitudinal Study, Kindergarten Class of 2010-11 (ECLS-K:2011), multilevel models are employed to integrate multiple sources of data from parent interviews, self-administered teacher surveys, school administrator questionnaires, and one-on-one student assessments. Drawing from research on family language policy, which focuses on how bilingual families manage and use languages, and on theories of bilingual education, this study shows that the academic support that parents provide using the home language gives bilingual children background knowledge or a network of contextual clues that helps them navigate English-only classrooms. This background knowledge gained through parental support in the home language allows bilingual learners with the lowest level of English proficiency to score higher in mathematics in English-only environments during the kindergarten year in the same way as home language support contributes to children’s success in bilingual and dual language programs. This finding is an empirical verification of what has been called “de facto” bilingual education, a situation in which an emergent English learner succeeds in an English-only classroom due to parental academic support in the home language. In addition, this study shows that the parents’ preference for a home language does not jeopardize the English language attainment of young children upon entry to kindergarten. These findings are of great significance to educators, policymakers, and researchers who strive for equitable educational practices that support the inclusion of all students in the classroom, as they provide a context for understanding oft-reported immigrant successes in English-only classrooms as “de facto” bilingual education provided by parents at home.