Crescendo is a music-based social and emotional learning (SEL) programme designed for primary/elementary school children living in disadvantaged communities. It is a community-led, orchestra-delivered, and evidence-informed initiative aimed at improving children’s musical and SEL outcomes through sustained engagement. Children growing up in socioeconomically disadvantaged areas often experience challenges with SEL and limited access to orchestral music education. However, emerging research suggests a relationship between music participation and SEL development. This study evaluated the initial impact of Crescendo on 559 children aged 5–6 in their first year of participation (Year 1 of 7). A quasi-experimental, rolling cohort design compared pupils in four participating Crescendo schools with pupils in four matched control schools not receiving the programme. Outcome measures included music skills (beat, pitch, and reaction to music) and SEL (behavioural self-regulation). The findings indicated significant positive effects of the programme across all outcome domains, with moderate effects observed in self-regulation (Cohen’s d = 0.29) and reaction to music (Cohen’s d = 0.21) compared to control schools. These results suggest that collaboration between orchestral musicians and educators can positively influence young children’s musical and SEL development in resource-constrained settings. The findings also underscore the importance of clearly defined programme models to support replication and scalability.