School music ensembles have the potential to provide a space where students can develop a strong sense of belonging and relatedness. A sense of belonging and relatedness has been shown to be an important factor in helping students avoid social isolation and its attendant issues of poor academic performance, lack of motivation, and behavioral problems. Particularly within a music education context, however, little is known about how fulfilling the need for belonging and relatedness might have a positive impact in the music classroom. In order to address this research gap, I used self-determination theory to test the hypothetical links among students’ perception of teacher support for relatedness, perceived relatedness satisfaction, general prosocial behavior, and music practice quantity and quality. I surveyed a sample of 749 high school band students about their perceptions of the band classroom and their band-related behaviors. Path analysis was then used to test the hypothetical model. As hypothesized, the findings of this study indicate a strong relationship between relatedness support and relatedness satisfaction. Results also show that relatedness support—through relatedness satisfaction—predicted certain general prosocial behaviors (compliant and public) and music practice quality. Also, relatedness need fulfillment was negatively associated with music practice quantity. These results indicate that teacher support for relatedness in band may play an important role in promoting other positive outcomes such as increased prosocial behavior and higher-quality music practice. This study also shows continued evidence for the viability of using self-determination theory to understand the motivational processes at work in the music classroom.