Re: Level of ability/Difficultly of subject matter or techniques
I'm considering organising art classes myself, so this is something I've also been thinking about.
It depends upon context of course, but I'm not sure I agree with your reasoning, e.g. that still life is easy and portraiture is hard, or that photorealism is harder than cubism? Maybe still life and cubism just appear slightly easier to get away with if they don't go quite right?!!
If I was to go back in time and have a proper art education, other than the one I had, I'd say that beginners should be taught foundations: observation, value, composition, perspective, proportion, etc and how to develop an 'eye'.
Intermediate would be doing the foundations well and getting good 'standard' results whatever the subject matter and learning some advanced techniques .
Advanced students would be masters of the basics and not only getting excellent results but adding their own flare or personality.
My concern with grading subjects rather than tools/techniques in order of difficulty is be that it might tend towards learning based on formula rather than observation; it's the difference between being given a fish or a fishing rod.
Just my opinion of course, I'm not an educator!