Language policy is generally seen as a national-level decision regarding which languages the state will support, and in which public domains. However, the reality is that language policy plays out at regional and local levels as well. In fact, it could be argued that the most important instantiations of language policy are those which directly determine local-language behaviors in institutions such as schools, government and civil society. Using data drawn from Kenya, this article examines the formulation and implementation of language policy as it plays out in the primary classroom environment. The relationships between language policy implementation at the classroom level and students' early literacy outcomes are explored, giving insight into how the degree of adherence to language policy in the classroom intersects with student achievement. The article presents findings using language use as a predictor, school and student-level economic status as control variables and student achievement as the outcomes. The country-level differences in language policy implementation between Kenya and Uganda, and the impact of those differences on student achievement in the two countries, are also examined. The article has implications for the establishment of a learning environment in the multi-language primary classroom, and demonstrates the extent to which choices about language policy implementation can present a serious challenge to effective education.