Most schools in Macao are private schools, and there is a variety of grade repetition policy practiced in the 45 secondary schools. The policies are translated into school-based accountability of some kind of minimum competency standards. The objective of this study is to uncover the mediation mechanisms accounting for the influences of grade repetition on student performance. Three mechanisms are postulated to explain the performance of the adolescent grade repeaters: (1) insufficient opportunity to learn, (2) inadequate self-regulation of students, and (3) inappropriate teacher guidance and management. Based on the Programme for International Student Assessment 2012 Study, the three identified mechanisms (involving eight mediation variables) together explain 0.7 grade level worth of schooling. The implication is that unless remedial programs and courses of action for the grade repeaters can be shown empirically to be effective, it is not a good idea for Macao schools to maintain their grade repetition policies without teachers’ proper guidance and instruction.