This cluster randomized trial evaluated the impact of the Promoting Alternative Thinking Strategies (PATHS) curriculum on children's psychological wellbeing, peer social support, and school connectedness. Forty-five schools in England were randomly assigned to implement PATHS or continue their usual provision for 2 years. The trial sample was 5,218 students, aged 7-9 at baseline (M = 8.12, SD = 0.87). Teachers in PATHS schools received initial training and ongoing implementation support and assistance from trained coaches. Multilevel intent-to-treat (ITT) analysis of outcome data indicated that PATHS led to a small, statistically significantly improvement in children's psychological wellbeing but had no discernible impact on their peer social support or school connectedness. Multilevel complier average causal effect estimation using dosage as a compliance marker increased the intervention effect size for psychological wellbeing and revealed significant medium to large effects for peer social support and school connectedness. The implications of these findings are discussed, and study limitations are noted.