This study compares the effects of private tutoring to the effects of in-school education on the academic achievement of high school students in Korea. The gain in popularity of private tutoring and out-of-school private lessons among Korean students is causing social and financial problems both for individual families and the country as a whole. Another problem is that as quality of public education in schools declines, students and parents have relied heavily on out-of-school private lessons for academic achievement. Thus, it is time to empirically determine the effects of both sectors of education on our students achievement, and evaluate them objectively, so that we can clarify our beliefs and concepts of their respective values. This study is based on the quantitative analysis of survey data collected by researchers using both a questionnaire administered to a nation-wide sample of 918 students from 30 general high schools, and a preliminary version of the Korean Scholastic Aptitude Test prepared by a major private testing institute. Multiple regression analysis was employed to estimate the effects of both sectors of education on performance, with appropriate controls of predetermining variables (e.g. sex, family background (parents education, income and cultural capital), personal characteristics (IQ, educational aspirations), and geographical region). The resulting data found that taking out-of-school private lessons did not appear to affect mathematics performance between the first and second achievement tests, while in-school education did have an affected on students mathematics performance.