If I wanted to become an Anatomy and Physiology High School teacher, what would I major in?
I imagine there are lots of routes that you might take, but ultimately I'd suggest majoring in something that interests you...
...but overall your best bet would likely be to major in a science such as biology, microbiology or kinesiology with an emphasis in secondary education. Then take as many elective anatomy and physiology classes as you can where time and funds permit. In addition, I strongly recommend reading up on schools that have human-dissection labs, because as a learning tool, it's unparalleled.
I'm not sure how much use this will be to you, so I'll keep it short; whatever you do, do it well! I wish you all the best.