We report on a nine-month-long observational study with teachers and students with autism in a classroom setting. We explore the impact of motion-based activities on students' behavior. In particular, we examine how the playful gaming activity impacted students' engagement, peer-directed social behaviors, and motor skills. We document the effectiveness of a collaborative game in supporting initiation of social activities between peers, and in eliciting novel body movements that students were not observed to produce outside of game play. We further identify the positive impact of game play on overall classroom engagement. This includes an "audience effect" whereby non-playing peers direct initiations to those playing the game and vice versa, and a positive "spillover" effect of the activity on students' social behavior outside of game play. We identify key considerations for designing and deploying motion-based activities for children with autism in a classroom setting.