Introduction and objective:
The physical, mental and social development that occurs in young people through physical activity (PA) is primarily through extracurricular activities. Family, peers and social environment, in addition to schools, interest groups and school sports, play a unique role during this developmental period. The objective of the study was to examine the differences in the intensity of PA during school days and weekends and the relationship between PA and physical inactivity (PI) during these days in Polish and Czech boys and girls.

Material and Methods:
In total, there were 816 participants among whom 333 met the requirements of 8 hours of continuous recording of PA (ActiTrainer accelerometers) during at least one school and one weekend day.

Results:
Boys and girls from both countries engaged in virtually the same amount of PA during school and weekend days, and participated in more PA at lower intensities on the weekends compared with school days.

Conclusions:
This study surveyed important issues related to global public health, specifically for the school environment and school settings. The important and crucial relations with family were emphasized, which should increase the awareness and understanding of public health problems of this particular research sample. The results indicated that less time was spent in PI, but also that the largest amount of time during the weekends was spent in front of a screen.