Background: This study sought to understand the relationship between school-based health centers (SBHCs) and academic outcomes such as early dismissal and loss of seat time (the time students are available in school to learn or to access support services). Methods: A quasi-experimental research design was used to compare rates of early dismissal and loss of seat time between students who received SBHC and traditional school nursing services and students who received only traditional school nursing services. This study was a secondary data analysis of 764 "walk-in" visits during a 3-week period in 2 urban high schools in western New York state. Both schools provided school nursing services, and 1 of the 2 offered the option to enroll in an SBHC. Results: SBHCs significantly reduced the number of early dismissals from school (p = 0.013) in a comparison with students who received school nursing services alone. Students not enrolled in an SBHC lost 3 times as much seat time as students enrolled in an SBHC. Race, gender, age, poverty status, and presence of a preexisting illness did not influence these findings. Conclusions: These findings suggest that SBHCs have a direct impact on educational outcomes such as attendance. Recommendations for further research include replication of this study to increase confidence in its findings and using early dismissal and loss of seat time as indicators of attendance to measure other health outcomes related to SBHCs and school nursing. (Contains 1 figure and 6 tables.)