The Assessment-to-Instruction (A2i) Professional Support System has been shown to be effective in improving literacy proficiency among young readers by combining data-driven technology with professional development (PD) activities to help teachers provide personalized literacy instruction. To identify efficient ways to expand the reach of A2i, this study uses a school-level randomized controlled trial design to assess the relative effectiveness of two PD models in implementing A2i and improving student literacy achievement. Fifty-nine schools in 20 school districts were randomly assigned to either a mixed-mode PD delivery model that combines in-person and virtual PD activities or a face-to-face model that is tested and resource-intensive. A cohort of 4,733 first-graders in these schools was followed and their reading achievement tracked for three years. Due to disruptions caused by the COVID-19 pandemic, the full implementation of A2i was not realized during the study period. As a result, while student achievement in reading was similar in mixed-mode and face-to-face schools across all three study years, such results might not reflect the actual relative effectiveness between the two PD models.