The aim of the study was to examine the mediating role of positive and negative affect in the relationship between Big Five personality traits (neuroticism, extraversion, openness to experience, agreeableness, conscientiousness) and school-wellbeing. Participants consisted of 474 that were selected among tenth and eleventh grades of Shiraz high school students using multi-stage random sampling and completed Big Five Personality Inventory (short form) (Khormaei & Farmani, 2014), positive and negative affect scale (Watson et al., 1988), and school well-being questionnaire (Konu & Lintonen, 2006). Findings applying structural equation modeling, showed that extraversion, openness to experience, and conscientiousness effects on positive affect were significantly positive. The effect of neuroticism on positive affect was negative and statistically significant, and on negative effect was positive and statistically significant. Moreover, neuroticism, agreeableness, openness to experience, and conscientiousness directly and indirectly with mediation of positive and negative affects could predict school well-being. According to this research findings, it can be concluded that increasing extraversion, openness to experience, and conscientiousness, and decreasing neuroticism in students can enhance their positive affect and school well-being.