Despite modest increases in high school graduation rates for all students, disparities still exist for some groups, such as students of color and students from low-income families who face systemic challenges in accessing high-quality public schools. Third-party "school support" organizations like the Urban Assembly have long focused on addressing this inequity. The Urban Assembly (UA) has been working with New York City schools since 1997. Currently, UA provides their schools with professional development to school-based staff using two formats--professional learning communities (PLCs) and coaching--focused on five main program areas: academics, leadership, social-emotional learning, postsecondary readiness, and alumni success. In addition, UA supports general school functioning in various ways, such as acting as a liaison to the New York City Public Schools (NYCPS) and providing hiring and staffing support. Currently, MDRC is conducting a study of the Urban Assembly to understand its model and its impact on student outcomes. This article is an initial discussion of UA's evolution which can be instructive for other organizations working to support schools to adapt in ever-changing contexts.