This five-year study focused on school processes that promoted the scaling-up of a high school academic literacy framework, Reading Apprenticeship, developed by WestEd's Strategic Literacy Initiative (SLI). Implementing an innovative strategy for scaling-up involving school-based cross-disciplinary teacher teams, SLI brought the framework to 274 schools across five states. Our study was guided by research literature that viewed scale-up as increasing local ownership and depth of commitment. Our goal was to measure SLI's success in terms of the numbers of students, teachers, and schools reached, and to evaluate the success of SLI's innovation through metrics that would be indicative of the program taking hold in schools, and ultimately, its scalability. We developed a longitudinal data set based on teacher and principal surveys, participant records, and school demographics that allowed us to measure implementation over time and to identify program and school characteristics that predicted the increase or decrease in participation. We found early indications of uptake and enthusiasm for Reading Apprenticeship, but greater variation in responses across schools by the third year. Using regression analysis, we found that initial teacher participation in team meetings and schoolwide commitment predicted "scaling-in" within a school. We hypothesized that a process, consistent with SLI's use of cross-disciplinary teacher teams, led to increased and sustained program participation. This study contributes to the field of scale-up research through identifying intermediate teacher outcomes that predict scaling-in and that were not as prevalent in an RCT conducted in parallel, illustrating the weakness of large-scale RCTs as an appropriate context for measuring impact of an innovation that is being scaled up.