Partial Comprehensive School Physical Activity Programming (CSPAP) has been studied extensively in recent years. However, there is little evidence on the efficacy and feasibility of such interventions in high school settings. Physical education teacher education (PETE) programs have been slow in preparing future physical educators for CSPAP implementation. Therefore, the purpose of this study was to assess the effects of PETE interns implementing a partial CSPAP intervention in high schools. Methods: Data were collected at three high schools during before- and during-school time periods using a direct observation instrument. The intervention consisted of PETE interns providing access to physical activity areas, equipment, and supervision with assistance from the schools' resident teachers. A hybrid multiple baseline research design was used to assess the effects of the intervention on high school students' participation and moderate to vigorous physical activity levels during the partial CSPAP sessions. Data were analyzed using both standard visual analysis of graphically plotted data and supplementary statistical treatments. Data were deemed credible based on interobserver agreement data collected across experimental phases and schools; the data were deemed trustworthy. Results: Experimental control was established as the total number of students engaged in moderate to vigorous physical activity during the partial CSPAP sessions increased substantially upon the start of the intervention. Compared with girls, boys demonstrated higher amounts of moderate to vigorous physical activity. Discussion/Conclusion: High school students respond similarly to a partial CSPAP intervention as do elementary and middle school-aged students, thereby strengthening the generalization of CSPAP-type interventions. Moreover, PETE interns can be successful in implementing a partial CSPAP in high school settings.