Following the adoption of food policies replacing unhealthy products by healthy foods in school, the present study tested the effectiveness of an intervention aimed at encouraging high-school students to stay in school for lunch instead of eating in fast-food restaurants.

A 12-week multi-strategy intervention targeting specific determinants of behaviour was evaluated via a quasi-experimental pre- and post-intervention design. A self-administered questionnaire was employed based on the theory of planned behaviour.

An experimental (n 129) and a control school (n 112) in central Canada.

High-school students aged 12 to 17 years.

Compared with control school students, those in the experimental school significantly increased the mean number of days that they stayed in school for lunch (relative risk = 1·55; 95 % CI 1·06, 2·27; P = 0·024), as well as the proportion who remained in school for lunch every day (relative risk = 1·21; 95 % CI 1·04, 1·40; P = 0·014). Among the psychosocial variables targeted, only self-efficacy appeared to be influenced by the intervention, mainly because of a decline in control group values. Mediation analysis indicated a significant mediating effect of self-efficacy on the mean number of days that students stayed in school for lunch (bias-corrected and accelerated point estimate = 0·079; 95 % CI 0·0059, 0·1958).

These results suggest that interventions aimed at enhancing self-efficacy can successfully contribute to students staying in school during lunch time. Such interventions should be considered in obesity prevention programmes adapted to high-school students.