I think, bigkid, that you either have very able classes or a very broad definition of profoundly special needs. I have one class I can teach like you describe, and it's great. Other classes the gap in knowledge is so large as to make even studying the same topic tenuous. How can you teach prime factorisation to a class when one member of it (with no known specific need) can't remember times tables? And not for want of trying, the poor lad had spent entire evenings drilling them with his mum, and then by the time he got up in the morning it was all gone again. He was an extreme example, but even if you have lazy kids and push them into working, they can't instantaneously make up for lost time; they can't leap in at the level of their ability, they have to catch up and build up knowledge and skills, which again requires differentiation.