Grade retention has been criticized because it reproduces existing social inequalities. Students with an ethnic minority status and/or an underprivileged socioeconomic background have a higher chance of being retained in comparison to their counterparts with similar school achievements. Grade retention can be problematic for the future academic career, as retainees have three to seven times less chance to obtain a degree in secondary education when accounting for school performance. It is still unclear, however, how sociodemographic factors such as socioeconomic status and ethnicity relate to teacher attitudes during the retention decision-making process. This research project is unique in studying whether teachers suffer from an implicit bias during this decision. An implicit bias occurs when gatekeepers unintentionally hold prejudices or stereotypes about people within certain categories, which leads to unequal outcomes because the bias alters the gatekeepers’ judgments and decisions. Flanders is an interesting case study for this topic, as it combines high retention rates with a high level of teacher autonomy. Hence, the Flemish educational system is particularly vulnerable to a potential bias of gatekeepers. The study will use a mixed-method approach, combining quantitative quasi-experimental video vignettes with qualitative in-depth interviews of teachers.