Adolescent mental health is a growing public health concern due to the high prevalence of mental disorders, many of which remain unrecognised and untreated. School staff are strategically positioned to promote mental health, recognise mental health problems and support pathways into care, but often lack sufficient mental health literacy (MHL) and confidence to act.

This study evaluated the effects of the WhySchool project, a school-based programme to promote MHL among teachers and school health professionals (SHPs).

We implemented WhySchool in 72 public middle and high schools across Portugal through a cascade training approach. With a pre–post design, we assessed 788 teachers and 201 SHPs on mental health knowledge (MHK), personal depression stigma, openness to seeking help and confidence in identifying/referring students. Paired-sample t-tests with Cohen’s d estimated changes, and generalised linear mixed models (GLMMs) accounted for confounders and within-subject variability.

The programme was associated with significant improvements in all outcomes across both professional groups, with moderate-to-large effect sizes (MHK d = 1.12 (95% CI 1.04 to 1.20); stigma d = −1.05 (95% CI −1.12 to −0.97); openness d = 0.44 (95% CI 0.37 to 0.51); confidence d = 0.87 (95%CI 0.79 to 0.94)). GLMMs confirmed these results. Gains varied across professional groups and demographic characteristics, with those having lower baseline scores generally benefiting most.

The WhySchool resulted in observable improvements in teachers’ and SHPs’ MHL, including increased knowledge, reduced stigma, improved help-seeking attitudes and strengthened confidence to support students. The cascade model provides a viable and sustainable strategy for large-scale implementation, empowering educational communities to better support student mental health.