That's true, but it puts teachers at a post-primary-school level in a real bind. It's hard or impossible to take a subject like physics or calculus (or even algebra, or English literature) and teach it from first principles in a semester or two, especially when most of the students are not very self-motivated or interested a priori in the work.

So what do you do when you have a class full of students who have already been basically failed by the system and who understand none of the foundations of your subject? If you fail them all, you're the one that is likely to get the ax, even if it's true that none of them understand your material. Most teachers resign themselves to doing their best and administering fairly traditional tests and classwork, so that hard-working students can pass even with no understanding.