The school is an obvious arena for interventions designed to promote mental health among children. A set of educational techniques named social and emotional learning, which focuses on students' self-control, social competence, empathy, motivation and self-awareness, has shown promising results in the United States. This is a study of the application of a similar method in Sweden (referred to as social and emotional training) for school years 2000/2001 through to 2004/2005. It is an effectiveness rather than an efficacy study, largely administered by school personnel, which relates duration of the training (1-5 years) to a set of outcomes previously found to be associated with mental health. Positive and significant effects were found on five of seven variables: internalizing problems, externalizing problems, mastery (reflecting self-efficacy or hopelessness), self-image and self-esteem and contentment in school. Effect sizes were medium. Somewhat surprisingly, no relationship was found between the intervention and the promotion of social skills. Nor was there any detectable long-term impact on bullying. Controlling for student gender did not moderate any of the effects.