This article investigates downgrading from the highest level of secondary education in The Netherlands. "Downgrading" is defined as continuation of secondary education at a lower level. We conduct a longitudinal case study of downgrading in the first 3 years of a secondary school. Univariate and multivariate statistical analyses are employed to identify the explanatory factors that lead to downgrading. These factors prove to be largely consistent with those mentioned in the literature on underachievement of intellectually gifted students, in particular the Munich model of giftedness and talent, suggesting this model may have wider applicability. Moreover, we identify factors that are already prevalent before students enter secondary schooling. This suggests that the academic chain should be the scope of study when we want to influence underachievement or downgrading in the educational system. The results of this study have been used as a basis for interventions in practice to reduce the extent of downgrading.