This study aimed at examining the effects of an outdoor educational intervention on the mental health of schoolchildren. Two elementary schools participated (N = 230); one experimental school where the intervention was implemented, and the other a reference school. Demographic questions and the Strengths and Difficulties Questionnaire were completed by the parents. An outdoor educational intervention was implemented at the experimental school, and the data collection was repeated after one year. The results point towards a small but non-significant improvement in mental health at the experimental school while adjusting for demographics. However, this effect was significantly moderated by gender: boys generally fared better than girls at the intervention school, relative to the reference school. The results indicate that it may be important to address gender issues when educational programmes are implemented in schools. (Contains 3 tables.)