Phase‐Specific Changes in Korean Adolescents' Health Behaviors and Mental Health Across COVID ‐19 School Closure and Reopening

Background The COVID‐19 pandemic disrupted school routines, but health changes during school reopening remain unclear. This study examined phase‐specific changes in health behaviors and mental health among Korean adolescents. Methods Nationally representative Korea Youth Risk Behavior Survey data (2018–2024) were analyzed across pre‐pandemic, pandemic, transition, and post‐pandemic phases. Outcomes included health risk behaviors, diet and physical activity, obesity, and mental health indicators. Weighted prevalence estimates and logistic regression models were used. Results The analytic sample included 376,066 adolescents (51.8% boys). Current smoking declined from 6.4% (95% CI, 6.1–6.7) pre‐pandemic to 3.8% (3.6–4.0) post‐pandemic, whereas breakfast skipping increased from 34.5% (34.0–34.9) to 41.6% (41.1–42.0). Although moderate‐to‐vigorous physical activity exceeded its pre‐pandemic level post‐pandemic, obesity prevalence increased from 10.9% (10.7–11.2) to 12.2% (11.9–12.5). Suicidal ideation decreased during the pandemic but peaked at 14.0% (13.6–14.5) during transition. Implications for School Health Policy, Practice, and Equity During reopening, schools should strengthen healthy meal and physical activity programs, sustain substance‐use prevention, and ensure equitable access to mental health screening and referral. Conclusions Recovery was uneven across health domains, identifying school reopening as a critical prevention window requiring sustained, phase‐specific intervention.