<p>Peer relationships are closely linked to academic development, but the distinct roles of best friend (BF) and fixed peer groups (PGs) across developmental stages remain unclear. Clarifying these patterns is important for designing age‑appropriate educational strategies in highly competitive academic contexts. We analyzed data from 5,480 youth aged 6–17 years in the 2022 Korean National Mental Health Survey for Children and Adolescents, a nationally representative cross‑sectional study. Participants were categorized as children (6–11 years) or adolescents (12–17 years), and academic performance—dichotomized as high (self-reported high or upper-middle grade) versus low (middle, lower-middle, or low grade)—was examined in relation to having a BF and belonging to a fixed PG using multivariable survey logistic regression. Models were adjusted for age, sex, household income, parental education, and urban/rural residence. Having a BF was associated with a higher adjusted prevalence of high academic performance in both age groups (children: 89.3% without versus 92.6% with a BF; adolescents: 91.4% versus 95.9%). In contrast, PG membership was associated with higher performance in children (90.7% without versus 93.0% with a PG) but slightly lower performance in adolescents (96.3% without versus 94.7% with a PG). These cross‑sectional findings cannot establish causality and may reflect bidirectional processes, but they suggest that the academic implications of peer relationships differ by relationship type and developmental stage.</p>