This study concerns expository writing in mathematics as well as the contribution of assessment strategies to the development of mathematics communication. We studied four 8th grade students (aged 12-13) working in a group, in order to perform three expository writing tasks, which were assisted by feedback and the use of supporting assessment documents (a script and an assessment criteria grid in the form of rubrics). Our findings suggest that there was some positive development in the students' expository writing throughout the study, particularly regarding interpretation and justification. The group of students was able to properly interpret what was asked of them, with reasonable levels of correction and completeness. Throughout the tasks, the group gradually included more relational justifications, instead of vague statements, rules, or procedural descriptions. Students used multiple types of representation. In general, further explanations were made through verbal language. The assessment strategies contributed to such development, despite some prevailing limitations.