This study investigates the academic trajectories of gifted high-achievers (HA) and underachievers (UA) in South Korea's magnet high schools, offering insights into the psychosocial and parental factors influencing underachievement in East Asia. Using a longitudinal design, the study analyzed grade point averages (GPA) and self-reported measures of academic interest, intelligence beliefs, and parental involvement among 104 12th-grade students. Hierarchical cluster analysis identified two groups: HAs, who exhibited consistent academic growth, and UAs, who experienced significant GPA declines during high school. Early academic interest in mathematics and science emerged as the key differential factor, with HAs demonstrating stronger engagement during elementary school--a foundation that might have supported sustained success in more demanding academic settings. Contrary to expectations, both groups reported similar growth mindsets and levels of parental involvement, challenging conventional assumption about their influence on achievement within Confucian-heritage cultures. The findings highlight the importance of early academic interest as a predictor of long-term success and point to critical vulnerabilities during transitions to higher academic demands, particularly within the high-pressure environments of magnet schools. Implications include the need for targeted strategies to nurture early engagement, enhancing resilience, and developing culturally responsive strategies to mitigate underachievement among gifted students.