(1) Background: Children's overweight and obesity are a growing public health problem. The aim of this study was to assess the influence of physical activity (PA) at school on body mass of children aged 10-12 during 2 years of observation. (2) Methods: Primary school children (n = 245, 48% girls and 52% boys) took part in the study. Children were divided in two groups, (1) of standard PA and (2) of elevated PA at school corresponding to 4 and 10 h of physical education lessons (PE) a week, respectively. Weight, height, and body mass index (BMI) were measured starting from the 4th grade and ending at the 6th grade of school. (3) Results: The number of children with excessive body weight (overweight and obese) increased by ¼ in children of standard PA while slightly decreased in children of elevated PA. Many more children of elevated PA changed body mass category from overweight to healthy weight than those of standard PA. Girls, especially of standard PA, had more often excessive body weight compared to boys. (4) Conclusions: Increasing time of physical activity at school by elevation of the number of PE lessons favorably affects the body mass of children.
Keywords: body mass index; children; obesity; overweight; physical activity.