Due to the increasing prevalence of adiposity in children numerous schools are introducing prevention programmes. Among these is “Gewaltlos Starksein” (“Being strong without violence”), a project of Hauptschule Sophienstraße Braunschweig, Germany (a general education secondary school for grades 5-10). This study aims to discover possible increases in activity through “Gewaltlos Starksein” where health–enabling technologies play a major role. A prospective intervention study with a span of 1.5 years was designed to measure this increase in activity. Partners in this study were Hauptschule Sophienstraße as the intervention group and Grund- und Hauptschule Pestalozzistraße as control group. Data collection was performed using a multi-sensor device, and questionnaires. Confirmatory data analysis of average metabolic equivalent (METs) yielded no significant results. Exploratory analysis showed interesting results, especially concerning the number of steps during leisure time. Descriptive analysis of questionnaires showed that all children enjoy physical activity. There were differences in sports team participation, open-air games and club affiliation. The study could not prove that the intervention “Gewaltlos Starksein” improves physical activity in children. However, the increased leisure activity step count indicates that “Gewaltlos Starksein” has positive effects on children's behaviour. This should be investigated in a further study in cooperation with psychologists.