School‐Based Social Relationships and Adolescent Mental Health: A Multilevel and LASSO ‐Based Analysis

ABSTRACT Background This study examined associations between school‐based social relationship quality (school belonging, peer support, teacher–student interaction) and mental health outcomes among high school students in Iran, identifying protective and risk factors for depression, anxiety, and stress. Methods A cross‐sectional survey was conducted in 2024 among 1200 students from 32 public and private high schools in Tehran‐Iran. Standardized measures were used to assess school belonging, perceived peer support from friends, teacher–student relationship quality, and symptoms of depression, anxiety, and stress. Data were analyzed in R using multilevel mixed‐effects models, quantile regression (50th and 90th percentiles), and LASSO‐based variable selection (glmnet). Results Higher school‐based relationship quality was associated with lower depression, anxiety, and stress. Effects were stronger in public schools than private schools: Depression β  = −1.727 versus −0.966; Anxiety β  = −1.513 versus −0.819; Stress β  = −1.601 versus −0.868. Academic stress was the strongest risk factor ( β  ≈ +1.60), followed by social media use ( β  ≈ +0.33). Socioeconomic status (SES) showed protective effects ( β  ≈ −0.72–0.81). Gender and age were minor; quantile regression showed stability. Implications for School Health Policy Strengthening school belonging, peer support, and teacher–student relationships, reducing academic stress, and supporting low‐SES students may improve mental health. Conclusion School relationships are key protective factors, with stronger effects in public schools, supporting school‐based interventions.