Reading, writing, and mathematics are widely regarded as foundational academic skills upon which many other academic skills depend. Consequently, each receives a considerable allocation of resources for instruction, assessment, and intervention in K-12 education (Hooper, 2002). An additional indicator of the importance of these skills is the federal mandate to assess students' proficiency in reading/language arts (including written expression) and mathematics each year in grades 3 through 8 and once in high school (Every Student Succeeds Act, 2015). To that end, schools are charged with the task of ensuring that all students graduate from high school with basic reading, writing, and mathematics skills. This article provides technically sound data regarding curriculum based assessment of written expression for use in a multitiered system of support.