As the 2015 revised national curriculum is being implementing in Korea, a teacher's assessment competency has become a point of focus for the potential enhancement of students' self-directed learning. Relevantly, a teacher's competency in student assessment is correlated to his/her individual feedback practices. Although students' self-directed learning affects academic performance and may vary based on their teachers' assessment competency, few studies have investigated the effect of a teacher's assessment competency, specially feedback practices, on student outcomes. Therefore, this study aims to empirically investigate the differential effect of a teacher's assessment competency, particularly focusing on feedback practices (i.e., formative assessment feedback, homework feedback), on Korean students' individual learning times and math achievements. For the analysis, a 3-level hierarchical linear modeling (HLM) is employed using data of middle school students (N = 2942) and math teachers (N = 426) in 207 middle schools of the Gyeonggi Education Panel Study (GEPS). The study revealed that a teacher's formative assessment feedback has a positive differential effect on math performance when controlling for student-, teacher-, and school-level covariates. In addition, the effect of individual learning time on math achievement was significantly dependent on teachers' homework feedback. The study provides newfound empirical evidence that teacher feedback plays a pivotal role in promoting a student's self-directed learning and academic achievement. Based on these findings, the implications of improving a teacher's assessment competency are discussed.