There are risks to too much sedentary behaviour, regardless of a person's level of physical activity, particularly for children. As exercise habits instilled during childhood are strong predictors of healthy lifestyles later in life, it is important that schools break up long sedentary periods with short periods of physical activity. Casual exergames are an appealing option for schools who wish to engage adolescents, and have been shown to provide exertion levels at recommended values, even when played for only 10 minutes. In this paper we describe a preliminary survey with teachers of a local school that informed the deployment of a casual exergame with a group of preadolescent students from the same school. We show that students preferred the game to traditional exercise, that the game was able to generate appropriate levels of exertion in pre-adolescents, and that students have a sophisticated understanding of the role of exercise in their lives. Overall, we establish the feasibility of casual exergames for combating sedentary behavior in preteen classrooms.