Shadow education or private supplementary tutoring has become an international phenomenon as increasing numbers of students seek help beyond traditional schooling for academic achievement. The positive relationship between students' academic achievement and participation in shadow education has been previously reported. However, the manner in which shadow education practices can help students to gain and maintain higher academic achievement remains understudied. In response, this study explored the features of shadow education practices that may benefit the academic achievement of students, particularly those in South Korea but with relevance beyond the South Korean context. Using qualitative research methods, this study revealed that preview learning from/with private supplementary tutors, academic mastery learning of subject knowledge and skills, training skills for school exams, and solving students' individual learning difficulties through intensive coaching are contributing factors to improving academic achievement.