In the face of Hong Kong's high grade-retention rates, this study aimed to investigate how Hong Kong secondary schools' grade-retention composition is associated with student performance and socioeconomic inequality in student performance. As the research questions involved analysis at the school and student levels, this study employed hierarchical linear modelling to analyse the Programme for International Student Assessment (PISA) 2018 data. While grade retention was often suggested to have a negative impact on repeaters' performance in studies using the same-age comparison strategy, this study found that a higher proportion of retained students at school was not associated with a reduction in students' performance. However, greater socioeconomic inequality in student achievement was found in schools with higher retention rates. In addition to providing plausible explanations for these findings, this paper discusses the potential role of the government's retention policies in these respects.