Writing in mathematics is complex. The purpose of this article is to share how one teacher, Ms. Hill, used peer-review activities involving rubrics to explicitly communicate mathematical resources that students could draw from when justifying a claim. She found that helping students understand which type of statements could be used in justification required consistent feedback and ongoing practice. The need for this consistent feedback and practice was especially true for the students described in this article. This case study was conducted in the lowest- tracked eighth-grade algebra class, and most of the students struggled mathematically. However, through peer-review activities, students learned to take an active role in evaluating both their peers' and their own arguments using a rubric, or assessment tool, that listed the criteria for a task.