To capitalize on professional expertise in educational assessment, it is desirable to develop and test methods of rater-mediated assessment that enable classroom teachers to make reliable and informative judgments. Accordingly, this article investigates the reliability of a two-stage method used by classroom teachers to assess primary school students' persuasive writing. Stage 1 involves pairwise comparisons and stage 2 involves rating against calibrated exemplars from stage 1 plus performance descriptors. A high level of interrater reliability among teachers was obtained. This is consistent with previous evidence that the two-stage method is a viable classroom assessment method without extensive training and moderation. Implications for assessment practices in education are discussed with a focus on the widely expressed desire to value professional expertise.