<p>This study examines whether baseline depressive symptoms and happiness predict trajectories of self-reported academic achievement among adolescents. Using five-year longitudinal data from the Korea Children and Youth Panel Survey (<i>N</i> = 2,471), we conducted latent class growth analysis to identify distinct trajectory patterns of self-reported academic achievement from 7th through 11th grade (2018–2022). We then examined whether depressive symptoms and happiness measured at baseline predicted membership in these trajectory groups, adjusting for gender, parental education, household income, urbanicity, parenting styles, peer support, and study time. Three distinct trajectories emerged: high-decreasing (29.3%), mid-gradually decreasing (59.4%), and low-U-shaped (11.3%). Higher baseline depressive symptoms predicted membership in the mid-gradually decreasing (OR = 1.47) and low-U-shaped (OR = 1.85) groups relative to the high-decreasing group, whereas happiness showed no significant associations. Household income, peer support, and time spent in private tutoring and self-study also differentiated trajectory groups, and the depressive-symptom associations were robust to these adjustments. These findings highlight the importance of considering mental health alongside socioeconomic and behavioral contexts in understanding adolescents’ academic self-perceptions, and provide insights for educators and policymakers developing targeted interventions.</p>