[charlesdm]

That's the question now, is it? It depends?

I had some horrible teachers in high school (one IT and physics teacher comes to mind). Most people flunked both subjects. I passed both, but only because I liked both subjects and had some affinity for them.

But he was definitely to blame: he understood the subject matter well, but was horrible at teaching and arrogant about it. I think they fired him a few years later.

If you have 85-90% of students failing your subject, something is probably wrong with the teaching (or the subject matter is too hard).

I think any kid should at least be able to pass a subject if he is of average intelligence + puts in some effort. Not too hard, but not too easy either. But perhaps also account for the fact that some people will always be horrible in one or two subjects.