This study underscores the importance of school-based assessment (SBA) in predicting students' mathematics performance in the Basic Education Certificate Examination (BECE) in Ghana. The study was conducted using 150 mathematics teachers from public junior high schools in Ghana's Central Region, the research employed a quantitative approach, using surveys to gather data on internal examination scores, SBA practices, teaching experience, and gender. Regression analysis revealed a strong predictive relationship between internal exam scores and BECE performance (R² = 0.663, B = 0.374, SE = 0.006, t = 67.24, p < .000), while ANOVA showed significant variations in SBA practices based on teaching experience (F (3, 146) = 6.99, p < 0.05). However, t-tests indicated no significant gender differences in SBA practices among teachers (t (148) = 1.100, p = 0.273). These findings highlight the critical role of internal assessments and teaching experience in enhancing educational outcomes and suggest that educational policies should support effective SBA practices and professional development for teachers. Based on these findings, it is recommended to enhance formative assessment practices by focusing on continuous and comprehensive internal assessments, provide ongoing professional development for teachers to improve their skills in implementing SBA, and allocate adequate resources for mathematics education to support effective teaching and learning environments.