Implementation of school meal guidelines is often inadequate, and evidence for effective implementation strategies for school-based nutrition interventions is limited. The aim of the present study was to examine the implementation and effectiveness of a multi-strategy implementation intervention to increase adherence to the Norwegian national school meal guideline.

The study was a school-based hybrid implementation effectiveness trial with a pre–post non-equivalent control group design, testing three implementation strategies: internal facilitation, training and an educational meeting.

Primary schools and after-school services in two counties in south-east Norway.

School principals, after-school leaders and class teachers from thirty-three schools in the intervention county and principals and after-school leaders from thirty-four schools in a comparison county.

There was a significant difference of 4 percentage points in change scores between the intervention and the comparison groups at follow-up, after adjusting for baseline adherence (B = 0·04, se B = 0·01, t = 3·10, P = 0·003). The intervention effect was not associated with the school’s socio-economic profile. School-level fidelity was the implementation dimension that was most strongly correlated (rs = 0·48) with the change scores in the intervention group, indicating that principals’ support is important for gaining the largest intervention effects.

A school-based intervention with low intensity, based on trained teachers as internal facilitators, can increase adherence to the national school meal guideline among Norwegian primary schools, irrespective of local socio-economic conditions. Implementation fidelity, at an organisational level, may be a useful predictor for intervention outcomes in schools.