This study focuses on the relationship between an autonomy-supportive classroom climate and changes in attitudes towards social participation in mixed-grade classes. The research field involved the learning activities conducted in a Japanese elementary school. Learning activities were carried out in mixed-grade classes, and the children engaged in problem solving with their peers from different grades in collaboration with the local community. It was hypothesized that a perceived autonomy-supportive classroom climate would affect attitudes towards social participation and that this effect would be mediated by intrinsic motivation and reflection. Participants were 163 third- and fourth-grade children from mixed-grade classes. Data were collected repeatedly over a three-year period through a questionnaire survey. Latent growth curve modeling analyses revealed that an autonomy-supportive classroom climate was related to intrinsic motivation, which in turn, was related to reflection. Additionally, intrinsic motivation and reflection predicted the initial level of attitudes towards social participation. Intrinsic motivation was negatively related to the extent of change in attitudes towards social participation. These results suggest that an autonomy-supportive classroom climate promotes children's positive attitudes towards social participation. In mixed-grade classes, teachers must create classrooms in which children can support each other's autonomy.