By 2013, all 50 states and the District of Columbia had adopted college- and career-ready standards in English language arts and mathematics, placing a greater emphasis on argument writing to prepare students for life after high school. Solving the specific problem of how to help teachers teach to new standards for argument writing as well as the broader problem of improving teaching and learning requires continued efforts to understand how to support teachers in making substantial changes in instruction. This paper reports on a district-randomized controlled trial of the National Writing Project's College-Ready Writers Program (CRWP), implemented in high-need rural districts in 10 states, testing one instance of professional development paired with supporting curricular resources and a formative assessment tool designed to shift instruction to align with the new college- and career-ready standards in English language arts classes in Grades 7-10. Researchers randomly assigned 44 rural districts either to receive two years of CRWP or to be in a business-as-usual control group and found positive impacts on teacher practice and student source-based argument writing.