In the context of intensified discussions on school consolidation amid a declining school-age population, this study examined the impact of elementary school size on students’ academic achievement using the 2024 Basic Academic Achievement Diagnostic Assessment results of 3,392 students in grades 3–5 from 36 schools in Gyeonggi Province. One-way ANOVA, χ² tests, and t-tests were conducted to compare academic achievement across school sizes (small, medium, large), grades, subjects, and genders. The results indicated that students attending medium-sized schools demonstrated the highest overall academic performance across most areas. In contrast, large schools showed superiority only in certain subjects, while small schools exhibited consistently lower achievement levels across all grades and subjects. Gender analysis revealed that female students outperformed males in Korean, science, and English, suggesting that gender differences are more sensitive to subject- and grade-specific characteristics than to school size. Overall, the findings support the optimal school size hypothesis, indicating that medium-sized schools provide a balanced learning environment conducive to stable achievement, while small schools require additional educational support to overcome structural limitations.