This article provides a replication study of the National Writing Project's College, Career, & Community Writers Program (C3WP), which was found to be effective at improving student writing quality in two prior studies. C3WP is designed to improve students' source-based argument writing through intensive teacher professional development, instructional resources, and formative assessment. In this study, we used a randomized controlled trial design, randomizing 22 of 44 rural districts into 1 year of C3WP for their grade 7-9 English language arts teachers. We collected argument writing from all students using a performance task similar to those in many state assessments, and we scored the writing for attributes of writing quality and conventions. We found that C3WP led to statistically and meaningfully significant findings on all attributes measured (g = 0.20-0.22). We interpret this study as validating the effects of C3WP on student argument writing quality and providing evidence that C3WP also positively impacts students' writing conventions. These three separate studies of C3WP spanned 228 diverse schools in 20 states, providing an unusually robust body of rigorous, independent evidence of program effectiveness.