In the ideal world with a realistic number of students, you’d assign something to write practically every day, you’d have conferences with all of your students, then they’d turn in first drafts and you’d make insightful comments and return them, and they’d consider your comments and hold workshop sessions with their peers, then crank out another draft and on and on until their essays were publishable.
You, unfortunately, are or soon will be teaching in a system funded by pocket change, so you’ll need to think more about surviving responsibly than actually teaching young people to write. It hurts me to acknowledge that you’ll basically wind up teaching “at” writing as opposed to teaching the thing itself.
Briefly, this is what you’ll need to do: Assign more essays than you can grade, and on those you do grade, find only one, certainly no more than two, areas to improve. More than that, and the student despairs. If there’s copious writing in the margins, the student either will not read it or, if she does, will either be pissed off or discouraged. She’ll go back to saying, “I’m just not a very good writer” or “I’m gonna get a bad grade in this class.” Neither response improves her writing. Be brief and direct and legible in your comments. When possible, let students revise essays based on your comments so that you know they read them, then speed grade the revisions, perhaps just tacking a few points on to the grades of the original.
As I grade a set of essays, instead of writing on several of them “You need to introduce quotes from the text, then briefly show the reader how it supports your thesis,” I jot down a brief note to myself about this issue. When I see other recurring errors or, yes, examples of effective writing, I note those too, and when the pile is complete, I put my notes on Blackboard, go over them in class and, when I feel the need, give quizzes on them. I have also asked them to revise their essays based on my Blackboard comments. I also refer back to this particular Blackboard document when it’s time for them to write another similar essay. This process saves much time and aggravation. (Obviously, it isn’t necessary to use Blackboard for this. It just makes it easy for all students to access this information at any time.)
You can assign a cluster of, say, three essays over a brief period of time, then have them run all three through the rubrics grinder, engage in peer editing if you’re into that scene, then pick the one they want you to grade. This way, they still get the practice, they aren’t penalized for their less successful efforts, and you only have to grade what is, according to them, their best work.