When students receive rubrics before completing the assignment, they understand how they will be evaluated and can prepare accordingly. Developing a rubric and making it available as a tool for students' use will provide the scaffolding necessary to improve the quality of their work and increase their knowledge.
The standards for excellence in a writing rubric remain constant throughout the school year; what does change is students' competence and your teaching strategy. Because the essentials remain constant, it is not necessary to create a completely new rubric for every writing activity.
