Adolescence is the peak life stage for the development of mental illness. Whole-school approaches to mental health and well-being, modelled on the World Health Organization’s Health-Promoting Schools Framework, hold vast potential in this developmentally sensitive period. However, the evidence base for these interventions is inconclusive.

Our study examines the effectiveness of The Resilience Project School Partnership Program, a whole-school intervention involving students, teachers and parents, centred around concepts of gratitude, empathy, emotional literacy and mindfulness.

A quasi-experimental study with an intervention and a control arm was used to evaluate the programme in 40 149 students across 102 schools in 2023. Data collected included sociodemographic information and outcomes derived from validated scales, comprising life satisfaction, hope, coping skills, anxiety and depression. Intervention schools were stratified by the number of years they had implemented the programme, and mixed-effects regression models were used to evaluate the programme.

After adjusting for confounders, participants at schools who had been implementing the programme for 6 years or longer demonstrated significantly better outcomes across all five domains (life satisfaction: B = 0.627, 95% CI 0.465–0.789; hope: B = 2.135, 95% CI 0.895–3.347; coping skills: B = 0.438, 95% CI 0.250–0.625; anxiety: odds ratio = 0.658, 95% CI 0.559–0.774; depression: odds ratio = 0.534, 95% CI 0.459–0.620). Only depression was significantly lower among participants at schools in their fourth or fifth year of implementing the programme (odds ratio = 0.941, 95% CI 0.935–0.948).

Our findings indicate that whole-school interventions may require long-term investment to realise their potential and highlight implementation duration as an important consideration for future evaluations of whole-school interventions.