There are two essays going on all the time – a practice essay and a regular "new" essay.
The practice essay is a single essay that grows all year long. Each time the students learn a new tool (invention, arrangement, or elocution) they practice the tool by adding the new element to the practice essay. This will give the teacher a chance to assess whether the student can use the new tool correctly and offer some feedback before he puts it in his current essay.
The practice essay is never graded. It is the place for first tries, experiments, and failure.
The students write a new essay about e new issue (in addition to and separate from the practice essay) for every lesson. So at the end of every lesson the students have TWO essays: the practice essay on which they've been practicing the new skills and the new essay with the new issue on which they've applied that practice.