The aim of this study was to analyse the relationships between physical activity level and anthropometric parameters, adherence to the Mediterranean diet (MD) and screen time in a representative sample of schoolchildren in Santiago. The girls showed spend fewer hours in front of the screen than boys (2.12 +/- 0.93 vs 2.64 +/- 0.93) and greater adherence to the DM (6.56 +/- 1, 97 vs 5.55 +/- 2.87). The students less active showed a higher BMI, triceps and subscapular skindfolds, waist circumference and fat percentage than the students more active. Likewise, the students less active spent more time in front of the screen. The most active students showed greater adherence to the MD (7.35 +/- 1.39 in the highest tercile vs 4.60 +/- 3.20 in the lowest tercile). It was observed as the level of physical activity was inversely associated with all anthropometric parameters, percentage of fat (r = -0.863) and subscapular skinfold (r = -0.875) showed the strongest correlations. We also found an inverse association between physical activity level and hours of screen (r = -0.875) and positive with adherence to the MD (r = -0.421). The hours of screen associated with higher values in all anthropometric parameters, percentage of fat (r = 0.793) and triceps skinfold (r = 0.791) those with the strongest correlations. In addition, more screen time showed lower values of adherence to the MD (r = 0.238). Greater adherence to the MD is inversely associated with all anthropometric parameters, with the subscapular skinfold (r = -0.397) and BMI (r = -0.380) those with the strongest correlations. Lower levels of physical activity are reported by students who spend more time in front of the screen and those further away from the MD, these students being those with a less healthy body composition. These results show the need to promote physical activity in this age group, with the aim of improving future health. They should be promoted policies of intervention to focus on promoting physical activity for the reduction of sedentary habits, as well as creating programs to improve the adherence to the MD.