I LOVE incorporating writing workshop into the classroom. Students can learn about the writing process and writing theory while still having the opportunity to be creative and to focus topics of their choice.
However, I do my writing workshop slightly differently. I assign (gasp!) the type of paper the students are going to write. Before you run away, let me just say how much this improved my writing workshop classroom. I was able to focus on particular writing theories as they applied to each type of assignment, so my students were learning how to write certain types of papers while still gaining the benefits of a writing workshop environment.
This includes my sixth and seventh grade writing workshop syllabi. Sixth grade wrote a short story (first quarter) and a personal narrative (third quarter). Seventh grade wrote a fairy tale (first quarter), a movie review (second quarter), and a research/novel analysis paper (fourth quarter).
Even if you choose to do your writing workshop differently, this handout could be a good jumping off point to constructing your classroom.