Mental ill-health is a major problem worldwide. It includes depression, aggressive behavior, feeling down and alcohol and drug abuse. Since all children go to school, the school is an obvious arena for health interventions. A set of educational techniques named Social and Emotional Learning, based on the use by teachers of cognitive and behavioral methods, which teaches students self-control, social competence, empathy, motivation and self-awareness, has shown promising results in the USA. This paper reports on the application of similar techniques in Sweden (the Social and Emotional Training [SET] program). The study has a quasi-experimental longitudinal design, with two intervention and two control schools. A wide range of instruments, both Swedish and international, are employed. In this paper, results from the school years 1999–2000 (baseline) through to 2001–2002 are reported. Both the intervention and the data collection were performed by ordinary school staff in a routine school setting. Independent bi-annual ratings of teachers’ performance were moderate to high, and teachers’ perceptions of the program were generally, although by no means universally, high. However, their performance was poorer with regard to the collection of data. In terms of promotion, findings with regard to the impact of the program on mental health are generally favorable—in particular through the promotion of aspects of self-image, including well-being and the hindering of aggressiveness, bullying, attention-seeking and alcohol use. There was, however, no differential effect on social skills. It seems that SET has the potential to operate effectively as a health-promoting intervention during the school period, although its main impact may rather be to act as a brake on the deterioration in some aspects of mental health that is common during adolescence. Positively significant relationships were found on some but not all of the instrument scales, and effect sizes were medium.