That is not always apparent to students. These five reasons along should justify the five or so minutes that it takes to bring closure to the Writing Block. If we're all starting new pieces at the same time, they can do a better job of supporting each other, and I can do a better job of monitoring what they're doing. However, kids will write best and most about what they know best and most about. In fact, we adults share the same sentiment.
Teachers can model writing daily and can teach a mini-lesson.
The majority of the time, however, after that introductory segment, the students should begin to write on topics of their own interests and choosing. Some will be starting a first draft, some will be continuing with a piece they previously started, some will be working on revisions and editing with the teacher, and some will be working on publishing their work. It takes some practice and skill on the part of the teacher for the workshop approach to work, but before long it runs like clockwork. The greatest reward is seeing the students take responsibility for their own work and pacing. Once the first student in the class publishes, all students want to get their work published.