Associations Between Body Mass Index, Physical Activity, Perceived School Competence, and Academic Performance in Portuguese Elementary Students

<b>Background/Objectives</b>: Childhood is a critical stage for consolidating health-related habits that shape physical, cognitive, and socio-emotional development. Regular physical activity has been linked to fitness and academic outcomes, while high body mass index (BMI) may hinder school performance. This study examined associations between BMI, perceived school competence, academic performance, and weekly physical activity volume in Portuguese elementary students, addressing a gap in national evidence compared with international studies. <b>Methods</b>: A cross-sectional correlational design was adopted with 531 children (M = 9.13 years) from 10 public schools. BMI was calculated from anthropometric measures, weekly physical activity was self-reported, perceived competence was assessed with Harter's Self-Concept Scale (Cronbach's α = 0.797), and academic performance was obtained from school records. Analyses included Pearson correlations, ANOVA with Scheffé post hoc, and multiple regression, with exact significance values reported. <b>Results</b>: Higher BMI was negatively associated with perceived competence and academic achievement, while regular physical activity, particularly 4-6 h per week, was linked to better results in Portuguese and mathematics. Associations were modest (r = 0.18-0.32; R² = 0.12). Regression showed physical activity (β = 0.093, <i>p</i> = 0.033) and perceived competence (β = 0.126, <i>p</i> = 0.004) predicted academic performance, whereas BMI was not (β = -0.028, <i>p</i> = 0.524). The near-zero correlation with environmental studies suggests subject-specific influences. <b>Conclusions</b>: Adequate BMI and regular physical activity are associated with better academic performance. The role of perceived competence is theoretically inferred as a potential mediator, but not formally tested. Findings highlight the interplay of physical, cognitive, and psychosocial factors. Structured school-based activity programs of 4-6 h weekly may promote both health and learning.