RESUMEN
School-based interventions for increasing physical activity among children are widespread, however there is still a lack of knowledge about how school context factors are linked to implementation quality and effectiveness of programmes. The aim of this paper is to examine teacher-perceived effectiveness of a Danish national classroom-based health programme 'Active Around Denmark' and in particular, to investigate whether perceptions vary as a function of school social context factors. After completion of the programme all teachers (N = 5.892) received an electronic questionnaire. 2.097 completed the questionnaire (response rate 36%) and 1.781 datasets could be used for analysis. The teachers were asked about their perceptions of changes in children's attitudes towards and levels of physical activity after the competition. Our results indicated that certain contextual factors, such as schools' prioritization of health promotion, teachers' support by their school principal in implementation as well as teacher's satisfaction with the school' physical environment made a significant difference in teacher-perceived effectiveness. To conclude, teacher-perceived effectiveness of the health programme does vary as a function of school social context factors.