This study examined the effects of shadow education on high school students' creative thinking and academic achievement in mathematics. Data from the Seoul Education Longitudinal Study were used for the analyses, including descriptive statistics, path analyses, and multiple group analyses. The results reveal that shadow education negatively influenced students' creative thinking in the long term, but positively affected academic achievement in mathematics in the short term. Furthermore, students who increased their hours of participation in the following year were more likely to present stronger effects of shadow education on creative thinking and weaker effects of shadow education on academic achievement. In considering the findings, mathematics educators, parents, and policymakers should not neglect shadow education when developing educational plans.