Background: Sedentarism is uniquely associated with numerous health problems (e.g., obesity). School-age youth spend a considerable portion of their time being sedentary, although relatively little attention has been given to examining youth sedentary time or behaviors during school.Purpose: This systematic review and meta-analysis examined sedentarism (time, behaviors) in children and adolescents during school hours. Methods: Two separate electronic-databases searches were conducted. The first focused on sedentary time and looked for studies that: (1) were conducted in the U.S., (2) targeted the K-12 setting during school-day hours, (3) were an intervention, (4) included objective measures, and (5) reported sedentary outcomes as a time-based metric. The second search focused on sedentary behavior and looked for studies that: (1) were conducted in the U.S., (2) targeted the K-12 setting during school-day hours, and (3) included reporting of sedentary behaviors. A pooled estimate of percent-time spent sedentary was calculated. Results: On average, youth spent 63% of their time in school sedentary. There were no studies that met the inclusion criteria for the sedentary behavior portion of the review. Discussion: The limited number of studies found that report sedentary-time, coupled with the absence of studies describing sedentary behaviors, suggests that further descriptive research is needed to understand school-based sedentarism in youth. Translation to Health Education Practice: Professional development for school staff and intervention work should encompass sedentary time and behaviors across the school day.