This study aimed to examine the predictive validity of junior high students' grit (including perseverance of effort (PE) and consistency of interest (CI)) on their short-term and long-term academic achievements using a longitudinal survey design under the structural equation modeling framework. Data were collected on 236 junior high students (56.3% boys) in Taiwan across three years. The analytical results showed that PE successively influences students' semester grades (as short-term achievements) in a unidirectional way. However, CI could not predict the consecutive semester grades. Furthermore, we found that both PE and CI did not directly predict students' achievement on the scores of the Comprehensive Assessment Program (CAP, as long-term achievements). Only PE can indirectly associate with the CAP scores via students' successive semester grades. The study results can be explained by the goal hierarchy of grit, where achieving good semester grades can be treated as the lower-order goals that are cohesively aligned with the superordinate goal to do well in the CAP. Strategies for fostering students' perseverance of effort and implications for future research on educational equity are discussed.