Background: Student discipline remains a persistent challenge in Ugandan secondary schools, affecting academic performance, school climate, and educational quality. While co-curricular activities, student age, and school rules have been independently examined as influences on behavior, their interactive effects remain underexplored in Sub-Saharan African contexts. Objective: This study examined the interplay of co-curricular activities, student age, and school rules in predicting student discipline among secondary school students in Uganda through development and validation of a comprehensive linear model. Methods: A cross-sectional survey design was employed with 384 secondary school students (S1-S6) selected through multistage sampling from twelve schools across four regions of Uganda. Data were collected using validated instruments measuring co-curricular participation (α=.82), school rules implementation (α=.85), and student discipline (α=.89). Hierarchical multiple regression analysis was conducted in four blocks, entering demographic controls, main effects, two-way interactions, and a three-way interaction term, with supplementary analyses including correlation, ANOVA, and simple slopes procedures. Results: Student discipline demonstrated strong positive correlations with co-curricular participation (r = .523, p