This study examines the role of shadow education as a contributor to educational inequality in South Korea. Using data from the Korean Educational Longitudinal Study (KELS), this paper analyzes the effects of private tutoring at 10th and 12th grade level on College Scholastic Aptitude Test (CSAT) scores. We reduce biases in prior research by using multi-time measures of private tutoring and instrumental variables approaches. Our findings show that shadow education increases the CSAT scores. We also find that shadow education significantly contributes to educational inequality even after controlling for various socio-economic characteristics.