Background: Physical education (PE) can provide opportunities to engage in daily moderate-to-vigorous physical activity (MVPA), but MVPA levels in many classes are low. This study examines MVPA during middle school PE lessons before and after receiving the SPARK (Sports, Play, and Active Recreation for Kids) program. Methods: Sixteen schools were enrolled in the study. PE teachers at eight schools received the intervention. PE lessons at all schools (N = 561) were observed over 2 years. Hierarchical linear regression models examined the effect of the intervention on the amount and consistency of MVPA and sedentary behavior. Results: An average of 13.7% of observed class time was spent in MVPA (approximately 5 minutes of a 60-minute class), compared to 27.5% of time spent sedentary. There was no evidence that the curriculum resulted in increased MVPA or consistent MVPA, or that it decreased sedentary behavior. Findings also suggested that contextual factors may contribute to physical activity levels in PE. Conclusions: Mixed evaluation findings of the SPARK middle school curriculum demonstrate that an out-of-the-box curriculum does not have the same results in all contexts. Implications for school health are described based upon findings. Further research is needed to identify effective strategies to increase MVPA for adolescents both in and outside of PE.