Competitive marching band festivals are among the most prominent performance opportunities for high school students in the United States. Yet questions remain about the equity of participation and advancement opportunities for bands from different communities. This study examined longitudinal patterns of participation and advancement in Bands of America (BOA) festivals from 2006 to 2024, focusing on how school-level demographic characteristics influenced participation and success. Archival data included 1,684 schools participating in 11,484 festival entries across 43 states. Descriptive analyses revealed that large schools consistently dominated participation. Over time, lower-poverty schools increasingly represented a greater proportion of participants, while higher-poverty schools became underrepresented. Logistic regression analyses indicated that school size was a stable, positive predictor of advancement, while school poverty was a more variable but powerful predictor, reducing the odds of advancement by as much as 92% in significant years. Two critical periods coincided with broader economic and societal disruptions, during which the disadvantages faced by high-poverty schools were further amplified. Findings underscore the role of structural and socioeconomic factors in shaping access to competitive music education opportunities for American marching band students. Implications include the need for support strategies targeting bands from higher-poverty communities.