Aggressive, violent, and disruptive behaviour among Belize youth is disproportionately high. The present study evaluated a school-based positive youth development program (Positive Action) using a clustered RCT. The aim of the study sought to examine post-treatment positive youth development competencies (i.e. social-emotional character development, peer affiliation, substance abuse and violence tendencies, moral beliefs, pro-social behaviour, and school self-esteem) in comparison to students in a randomly assigned control group of schools. Participants (n = 4575) were from 24 schools drawn at random in the Belize District with outcomes assessed for two cohorts (Standard 1-3, and Standard 4-6) of children. The intervention addressed different ecologies through teacher, counsellor, and family training along with activities to promote school-wide implementation. Positive main effects for engagement in positive behaviour were observed across primary school Standards 1-6 (ages 7-12 years), indicated by substantial explained slope and intercept variance.