Earlier quoted context omitted.
If the new material built on the old, shouldn't the old material be a solid lock?
Only then, and most courses are not straight progressions. In high school and particularly AP, those courses are blatant cycles of 1) cram, 2) test, and 3) forget all but the core of what that section was about. (If you want to say, "You should learn it all for the final" keep in mind nobody does , nor are final exams built around remembering everything from the whole course.)
The great thing about AP classes was that the teachers could really teach, and they could justify themselves by pointing to the AP test if anyone complained about the rigor of the work. The students were game, too, at least the ones who didn't drop the classes. Everybody sincerely tried to understand what they were doing instead of just trying to make the right pencil marks on the test paper, since understanding was the most efficient way to retain something for months at a time. The test looming at the end of the year rendered cramming pointless -- you could cram for one test, but you would have to eventually learn the material anyway in order to take the AP exam, so cramming for a test actually felt like the failure that it was.