Purpose <p>School Connectedness (SC) has increasingly attracted attention in research on the promotion of adolescent mental well-being and the prevention of mental health problems. However, related constructs (e.g. school belonging), often show conceptual overlap, leading to the development of multiple instruments for assessing SC. Furthermore, few studies have examined the associations between SC and both positive and negative mental health outcomes. The present study addresses this gap by validating the School Connectedness Scale (SCS) and examining its associations with a range of mental health indicators, including quality of life.</p> Methods <p>A total of 621 adolescents aged 11–18 years and enrolled in public schools, completed self-reported measures assessing SC, social safeness, mental well-being, anxiety, depression, and quality of life. Analyses included factor structure, internal consistency, measurement invariance, sex-based comparisons and construct validity. Additionally, associations between SC and mental health outcomes were explored.</p> Results <p>The original 5-item, one-factor model showed poor fit, which improved following examination of modification indices. Removal of item 4 yielded a revised 4-item model with good fit indices. This model was invariant across sexes, with males reporting higher SC than females. The SCS showed good internal consistency. Correlations with both positive and negative mental health dimensions were in the expected directions. SC showed significant cross-sectional associations with positive mental well-being, quality of life, and social safeness.</p> Conclusions <p>This study provides evidence supporting the use of the revised 4-item SCS with adolescents and highlights its potential as a valuable tool for future research examining SC and its association with mental health outcomes.</p>