Schools are recognized as one of the most important settings for promoting social and emotional well-being among children and adolescents. This clustered randomized controlled trial evaluated Zippy's Friends, an international school-based emotional well-being programme, with 766 children from designated disadvantaged schools. The purpose of this study was to evaluate the immediate and long term impact of the programme and to determine the impact of implementation fidelity on programme outcomes. Teachers reported emotional literacy outcomes using the Emotional Literacy Checklist, and emotional and behavioural outcomes using the Strengths and Difficulties Questionnaire. Controlling for the hierarchical structure of the data, path analysis using structural equation modelling revealed that the programme had a significant positive impact on the children's emotional literacy scores including significant improvements in the subscale scores of self-awareness (P < 0.001), self-regulation (P < 0.01), motivation (P < 0.001) and social skills (P < 0.001) at post-intervention. These results were maintained at 12-month follow-up (P < 0.01). The programme, however, did not have a significant impact on children's emotional and behavioural problems. Analysis of programme fidelity indicated that high fidelity was directly related to improved emotional literacy scores at post-intervention.