Today's college and career-ready standards emphasize developing students' ability to interpret and analyze complex texts and to assert and defend claims relating to those texts in extended pieces of writing. By providing secondary English language arts teachers with professional development and instructional materials, the UC Irvine Writing Project's (UCIWP's) Pathway to Academic Success program (Pathway) seeks to support students to meet their state-adopted English language arts standards and graduate from high school prepared for college and work. In 2018, based on evidence of prior success in improving student writing, UCIWP received a federal Education Innovation and Research (EIR) Expansion grant to extend capacity to deliver Pathway teacher professional development in new contexts and validate the efficacy of the approach in these new settings. This technical report describes a school-level randomized controlled trial of Pathway, implemented by UCIWP and seven expansion Writing Project sites in seven states (AZ, CA, MN, OK, TX, UT, and WI). Independent SRI researchers randomly assigned 46 secondary schools to Pathway or a business-as-usual comparison group. In the Pathway schools, English teachers serving students in grades 7 through 11 participated in 1 or 2 years of professional development. SRI researchers measured student outcomes on the Analytic Writing Continuum for Literary Analysis (AWC-LA). Despite disruptions to program implementation during the COVID-19 pandemic, the study found positive effects on student writing quality, conventions, and productivity. The size, rigor, and independence of this study provide a strong evidence base to support Pathway's effectiveness in improving secondary students' academic writing at scale and in diverse contexts.