Making sense of fractions is critical for building the mathematical competence of upper elementary students with and at-risk for a mathematics learning disability. The primary aim of this study was to investigate the effectiveness of teaching students with and at-risk for learning disabilities an intervention in which they learned to construct written arguments to develop their fraction knowledge. We sequenced the intervention using the 6 stages of Self-Regulated Strategy Development (SRSD). The effects were tested using a pre-posttest cluster-randomized controlled trial in which 10 teachers were randomly assigned to treatment and control conditions. Measures included a far-learning fraction test and proximal writing measures that assessed quality of mathematical reasoning, total number of rhetorical elements, and total words written. Teachers in the treatment condition received 2 days of professional development before implementing the six-lesson intervention, 3-4 times per week for 30-45 min. Clustering effects were corrected to examine differences in pretest-to-posttest change scores using individual scores for students in the SRSD group (n = 28) and control (n = 31). Results favored students in the SRSD condition from pre- to posttest on fraction test (g = 0.60); quality of mathematical reasoning (g = 1.82); number of rhetorical elements (g = 3.20), and total words written (g = 1.92). Special education students in the SRSD condition (n = 16) demonstrated greater gains in fraction scores from pretest to posttest compared to their nondisabled peers (n = 12, g = 1.04). The findings from this study support the genre knowledge hypothesis of writing-to-learn.