Academic self-concept has received a great deal of attention in recent educational research because it mediates many other educational outcomes. Therefore, it is important to find out how students' academic self-concept develops. We examined the big-fish-little-pond effect (BFLPE) dynamically by investigating the effect of track changes in high school on changes in students' academic self-concept. A sample of 2,747 students from 50 high schools in Flanders (Belgium) was longitudinally followed up in a study with a four-wave design. Data were collected in Grades 7, 8, 10, and 12. Multilevel growth modeling for discontinuous change was performed. The results showed a general decline in academic self-concept across high school. In line with the BFLPE hypothesis, students who changed from a higher to a lower track during high school experienced an initial increase in their academic self-concept compared with students who remained in the same track. This effect was neither attenuated, nor exacerbated over time. These findings extend the BFLPE research by revealing that students' downward changes in educational reference groups over time (cascade effects) are generally related to upward changes in their academic self-concept over time. However, track dropping seems a double-edged sword because students' downward track changes simultaneously resulted in decreases in academic achievement. The results of the current study therefore call for more longitudinal and dynamic research in both the BFLPE and educational effectiveness literature. (Contains 5 tables, 2 figures and 7 footnotes.)