Using a national sample of students' writing from the National Study of Writing Instruction, this study examines the current state of writing in the mathematics classroom and the extent to which enactments align with the intentions established in the research base and curricula articulated by NCTM and the CCSS-M. Over 3,000 writing assignments from 138 students in grades, 6, 8, 10, and 12 were analyzed for functions of writing and epistemic complexity--the degree to which they explained or justified mathematical understanding. Additionally, shortanswer assignments were analyzed qualitatively for patterns in teacher-assigned prompts and student responses. Analyses reveal enactments of writing in the mathematics classroom do not align with the intent articulated by the CCSS-M. Mirroring techniques from the literacy field, a method for scaffolding students' improved ability to write about conceptual understanding in mathematics is discussed.