Welcome to HW!
I am a teacher also, and I take remicade. I can do mine as late as 2:00, so you would only need a sub for a little bit. I don't see why that wold be a problem. And after your first three infusions, you only go every 8 weeks usually, so that's not a big deal. If your employer can't be understanding with you having to miss a couple of hours every 8 weeks, then that is no place that I would want to work!