Background: The transition from lower-secondary to upper-secondary education marks a critical period for adolescents that impacts their academic achievement. Objective: Based on Positive Youth Development (PYD) framework, we examined associations between math achievement as measured by PISA (Programme for International Student Assessment) and PYD outcomes, such as Character, Confidence, Connection, and Caring, as well as variability by gender and school types. Method: Using a cross sectional study design, we investigated Slovenian adolescents (N = 2802; 52% girls) who recently transitioned to upper-secondary level, who had participated in PISA study, and who had completed a PYD Short Form questionnaire. Results: We used multiple regression analyses to address the study goals. Significant positive associations between Character, Confidence and math achievement and significant negative associations between Connection and math achievement were observed. In general, the associations between Caring and math achievement were not significant. Specifically, Confidence was positively associated with math achievement in general gymnasiums, in technical programmes, and among boys in our sample. Caring was negatively associated with math achievement in technical gymnasiums. Connection was negatively associated with math achievement in technical programmes. Character was positively associated with math achievement in vocational programmes of medium duration. Conclusions: Across school types and gender, different associations were significant, indicating the importance of contextualizing when planning interventions. The results support the need for contextual understanding of factors that can promote academic (math) achievement. As Slovenia is ethnically homogeneous, the effects of contexts, such as school types and gender, can be given more attention as key factors for promoting PYD.