So I am working split between a flexible position and an on call position as a substitute teacher/aide in the same organization, at a school that teaches blended K-8.

I am completely out with administration, and they are working out the kinks on adding the new nondiscrimination policy and such. They already talked with department A, the flexible one, and I am meeting in a few days to discuss my transition etc.. they have already changed pronouns for me. Then the school's head teacher, who had perfect attendance, gave notice. There is no new teacher yet. That leaves me working a LOT at the school.

The first thing that happened when I walked in was that the staff started talking to and about me in a hurried strategy meeting off to the side of the classroom (into which a couple of kids who are early are already sitting), using lots of "he" and "Mister". I cringed a lot. However, I haven't hit the point where I am correcting people yet in that group, because my coming out hasn't actually gotten there and well, an emergency meeting just didn't seem like the greatest time for that discussion.

But it's going to come up at some point. I've been on estrogen for almost a month. My husband is going to be starting T sometime this month. Staff in places I am out have kids in that class. And I am pretty annoyed at the misgendering! But I have NO idea how this should be approached as far as the class is concerned. Has anybody seen any situations that are even remotely similar, any issues or thoughts or existing policies that people have seen?

It's not clear from your post, but if you aren't presenting you are more likely to be misgendered. If you are presenting, then each class that you handle will need an introduction the the correct behavior and you may want to include some time for Q&A. You are a teacher so this is just like any other subject you have taught but you need to make your point in part of an hour.

Well, at the moment, I can't really present successfully, without dressing far, far more high femme than ANY of my coworkers. And then I pretty much look ridiculous, and force the issue. Plus, I don't own enough clothes yet. *sighs* I've actually worn a short dress and leggings there and nobody seemed to really notice.
Today was shirt, pants, very light cosmetics, pretty similar to the other women there. The only guy on staff there actually had more makeup on than I did, and is often GNC.