To refine selective admission models, we investigate which measure of prior achievement has the best predictive validity for academic success in university. We compare the predictive validity of three core high school subjects to the predictive validity of high school grade point average (GPA) for academic achievement in a liberal arts university programme. Predictive validity is compared between the Dutch pre-university (VWO) and the International Baccalaureate (IB) diploma. Moreover, we study how final GPA is predicted by prior achievement after students complete their first year. Path models were separately run for VWO (n = 314) and IB (n = 113) graduates. For VWO graduates, high school GPA explained more variance than core subject grades in first-year GPA and final GPA. For IB graduates, we found the opposite. Subsequent path models showed that after students' completion of the first year, final GPA is best predicted by a combination of first-year GPA and high school GPA. Based on our small-scale results, we cautiously challenge the use of high school GPA as the norm for measuring prior achievement. Which measure of prior achievement best predicts academic success in university may depend on the diploma students enter with.