We do essentially independent study with AoPS. My son (5th grade) still has to take the tests in regular math, which is a breeze, but he's allowed to work on AoPS when the rest of the class does their math. The teacher will help my son if he gets stumped. I think it's an ideal situation. AoPS is much more rigorous that what they'll be teaching in a regular 6th grade class, so if your son is already doing it, I would ask to continue.