Background: This systematic review determines the levels of physical activity (PA) during school hours in children and adolescents. Methods: Studies carried out from January 1987 to December 2019 were retrieved from four databases (Web of Science, Pubmed, Scopus and SportDiscus). Results: Twenty-nine studies were included in this systematic review. Most of them used accelerometers and showed that male and female children accumulated a mean of between 14 and 68 min of moderate-to-vigorous PA (MVPA) during school hours (3–22% of this daily segment), and male and female adolescents accumulated a mean of between 13 and 28 min of MVPA during this daily segment (3–8% of the school hours). Less than a quarter of children and adolescents reached the recommended 30 min of MVPA during school hours, with notable differences between sexes. Conclusions: These results suggest that the levels of PA during school hours are not enough, and consequently, schools should develop strategies for helping children and adolescents reach the school PA recommendation.