This isn't the question you asked, but have you asked the high schools in the areas where you want to teach what course of study you need to follow in order to become a high school teacher?
In my corner of the world, I believe you need to get a bachelor's degree in education with at least a minor in math, complete a student teaching semester at a high school in math, and obtain some sort of state certification. I don't think the Khan classes would count towards any of that, although it may be a good brush up before starting a college degree in that direction.
I agree with seattlecyclone's link about discrete math. As a (possibly helpful) analogy, discrete is like turning the light on and off whereas continuous is like turning a dimmer switch.