Gosh, I'm sorry you're having these health problems. It definitely sounds like some kind of adrenal burnout/thyroid issues, what with the fatigue, light headedness, nausea and shaking. Being a teacher myself (elementary though instead of high school) I can empathize. You can't just zone out when in front of a live audience!
I wrote this post three years ago, and got quite sick the two years following. I'm finally starting to feel better now after starting Nutritional Balancing through hair mineral analysis, and sorting through some hidden traumas that had gotten me in a position of high stress in the first place.
Keep looking and searching for answers...they are there, and people do recover!