Background: This study investigated the associations of subjectively and objectively measured physical activity (PA) and sedentary behavior with academic achievement. We further examined whether aerobic fitness, obesity, and bedtime mediate these associations. Methods: This study included 970 children aged 9–15 years (52.3% girls) from 9 schools throughout Finland. Register-based academic achievement [grade point average (GPA)] as well as self-reported and accelerometer-measured PA/sedentary behavior were assessed during spring 2013. Aerobic fitness (assessed via a maximal shuttle run test), body composition (assessed via bioimpedance analysis), and self-reported bedtime were collected. Structural equation modeling was applied to examine the associations. Standardized regression coefficients are presented. Results: Self-reported PA had a direct positive [β = 0.084; 95% confidence interval (CI), 0.023 to 0.145] and an indirect positive association with GPA through higher aerobic fitness (β = 0.061; 95% CI, 0.033 to −0.087). Accelerometer-based PA was not associated with GPA. Self-reported screen time had an indirect negative association with GPA through later bedtime (β = −0.071; 95% CI, −0.096 to −0.035) and lower aerobic fitness (β = −0.039; 95% CI, −0.059 to 0.019). Nonscreen sedentary time had a direct positive (β = 0.193; 95% CI, 0.101 to −0.289) and an indirect negative association with GPA through lower aerobic fitness (β = −0.040; 95% CI, −0.063 to −0.016). Conclusions: Participating in PA, avoiding excessive screen time, and going to bed earlier may benefit academic achievement.