In this study, we examined the longitudinal effect of the free semester system on middle school students' academic achievement and career maturity using the Korean Education Longitudinal Study data, cohort 2013. We performed Difference-in-Difference (DiD) method to analyze whether the free semester system has effects on academic achievement and career maturity using the model moderated by parental academic support. We furthermore utilized quantile DiD method to examine whether the effects of the free semester system differ across the distribution of academic achievement and career maturity. As a result, we first found the significant positive effect of the free semester system on middle school students' academic achievement and career maturity. Second, we found that parental academic support mediated its effects on math performance. Third, our findings indicate that the free semester system improves students' academic achievement and career maturity, especially among students who performed to the bottom of its distribution. Based on these findings, practical implications and future research are discussed.