Purpose – Within the framework of Health Promoting Schools Up is an intervention using a whole school approach aimed at promoting mental health by strengthening social and emotional competence among schoolchildren. Social and emotional competence is an integral part of many school-based mental health interventions but only a minority of interventions measure changes in competences. The purpose of this paper is to present the intervention Up and document changes in social and emotional competence among schoolchildren before and after the intervention. Design/methodology/approach – Up consists of four components: education and activities for schoolchildren; development of staff skills; involvement of parents; and initiatives in everyday life at school. Up was implemented in two Danish schools in 2010-2011. Social and emotional competence was measured among 11-15-year old schoolchildren before (response rate 96.2 per cent, n=589) and after (response rate 83.9 per cent, n=532) the intervention. Findings – Changes in level of social competence were assessed by the prevalence of a high level of social and emotional competence before (33.3 per cent) and after (40.8 per cent) the intervention (p-value=0.01). Research limitations/implications – Up provides valuable experiences for adapting evidence-based mental health promotion to the Danish school system which is characterized by democracy, autonomy and inclusion. Future research should study the implementation and effect of Up in larger scale studies. Practical implications – The comprehensive description of Up serves as important information for policymakers and practitioners working with mental health promotion. Originality/value – The whole school approach intervention Up has the potential to promote social and emotional competence and reduce socioeconomic differences in social and emotional competence among schoolchildren.