Using data from the Korean Educational Longitudinal Study(2013), this study examines the impact of school size on admission to top-tier universities. Using piecewise linear regression and Instrumental Variable(IV) analysis to estimate the effect of school size on admission rates, the results indicated that larger schools are more advantageous for gaining admission to elite universities. Particularly, students from small schools were found to have about half the likelihood of being admitted to top-tier universities compared to those from medium and large schools. However, the effect of school size became statistically insignificant when the school size exceeded approximately 300 students(+1SD), according to the piecewise linear regression results, suggesting that the size effect diminishes beyond this point. The results from IV analysis, which corrected for endogeneity errors, consistently showed that larger school sizes favorably affect the admission of students to top-tier universities, especially those applying through transcript based admission system and those with high academic achievers. Based on these findings, the researchers discussed the possibility that school size could exacerbate regional educational inequalities in conjunction with various educational systems, such as university admissions systems, high school credit systems, and norm-referenced grading methods.