Most parents consider private supplementary tutoring (PT) as an effective means of improving academic achievement. However, previous studies on this method have produced either partial or inconclusive results on its effectiveness. Thus, in the present study we conducted a comprehensive analysis of the learning of middle school students in China, with specific focus on the final year of middle school. The analysis was based on a specially designed longitudinal survey of private supplementary tutoring in mathematics. An analysis using hierarchical linear regression showed that regular private tutoring, throughout the school years, could have a minor effect on students' mathematical achievements by the final year of middle school. These results suggest that parents should make careful choices for their children's private tutoring, and the government must issue comprehensive, professional guidelines to regulate the private tutoring industry. Moreover, other countries could find major take-aways from the Chinese experience of private tutoring for enhancing students' mathematical performances.