When I was in high school there were AP courses, and you had a choice - if you were in a school that had them - whether to take them or not. I was out before No Child Left Behind, though I saw or ran into two problems while I was in school:
Not every school had AP courses, or even good courses, and the quality of even the regular courses (and teachers) varied greatly in different schools,
And, if you failed to learn something, or forgot it, or it was never taught in the first place, and it turned out to be critical to a later course, you could end up being screwed by the time you realized.
It didn't help that our entire education system was (still is?) set up in a way that it seems to discourage permanently learning things. The summer break doesn't help. I won't guess about testing since that all changed with NCLB and will all be changing again. I had another reason, too: With math, for instance, if my teachers couldn't explain to me how anything was useful, I had no real incentive to learn it.
I remember in high school I often solved things using methods other than the way I was supposed to do it, too, because I saw a way to do it with what I already knew without needing to learn and memorize a bunch of new stuff that the book/class was trying to get us to learn. So I'd get the right answers and get full credit, but not actually learn what I was supposed to be learning. Of course that did mean that I continued to remember how to use the methods that I was using, whereas anything I learned and used just for a week would tend to be forgotten, so...