Background: This study was designed to model the relationship between an ActiGraphbased “in-school” physical activity (PA) and the daily one among children and to quantify how school can contribute to the daily PA recommendations. Method: Fifty boys and 43 girls (aged 8 to 11 years) wore ActiGraph for 2 schooldays of no structured PA. The daily moderate-to-vigorous PA (MVPAd) was regressed on the school time MVPA (MVPAs). Then, a ROC analysis was computed to define the required MVPAs. Results: Children spent 57% of their awaking time at school. School time PA opportunities (ie, recesses: ~18% of a child’s awaking time) accounted for >70% of the MVPAd among children. Then, MVPAd (Y) could be predicted from MVPAs (X) using the equation: Y = 2.06 X0.88; R2 = .889, P < .0001. Although, this model was sex-specifically determined, cross-validations showed valid estimates of MVPAd. Finally, with a sensitivity of 100% and a specificity of 90%, MVPAs, a 34 min.d−1 was required to prompt the daily recommendation. Conclusions: The current study shows the contribution of MVPA at school to recommended activity levels and suggests the value of activity performed during recesses. It also calls for encouraging both home- and community-based interventions, predominantly directed toward girls.