Many scholars have investigated the persistent poor performance of Nigerian students in English, particularly at the secondary and tertiary levels. However, little attention has been paid to foundational factors at the early stage of language development. Therefore, this study was designed to investigate two critical issues: (1) the use of non-complaint English Readers and Literature texts in primary schools and (2) the widespread violation of Nigeria’s language policy, which recommends initial literacy development in the mother tongue before the introduction of English in selected public and private primary schools in Ondo West Local government. Cummins’ interdependence hypothesis and Vygostsky’s socio-cultural theory, which both emphasize the centrality of a strong first language foundation for effective second language acquisition and cognitive development, served served as the framework. Adopting a qualitative approach through policy analysis and classroom-based evidence, this study argues that premature immersion in English and reliance on inappropriate instructional materials disrupt the transfer of linguistic and cognitive skills from the first to the second language. This weakens the overall language competence of learners and contributes to sustained academic underperformance. The findings reveal a significant gap between policy prescriptions and pedagogical practices in Nigerian primary schools. This study concludes that this trend can be reversed by the effective implementation of mother- or tongue-based instruction and the strict regulation of instructional materials to foster bilingual competence and improve the performance of students in English. Therefore, this study recommends renewed policy enforcement, teacher training, and stakeholder awareness as critical steps toward addressing the root causes of the problem.