I think we have to abandon the concept of grade levels by age to make progress in education.
The base curriculum should be the "conveyor belt" for the kids with the teachers moving the kids along the curriculum at their pace.
These programs, and compacting curriculum in general, work very well with math because it is a well-structured field. There is tremendous agreement about the order in which students should learn bits of math. You must learn this before you can learn that. If you have got fractions down, you are ready to learn X, Y, & Z.
Self paced becomes problematic in subjects that are not as well structured.
This is why you need a curriculum as a structure. These exist in all domains - every field has several structured ones and the homeschool suppliers have it down pretty good.
If you find a curriculum is confusing then more than likely the author has an ideological basis for making it confusing.
