What you're proposing is exactly what my 5th grade son is doing right now. He was already skipped in math to 6th, but it was too easy, so we were able to sign him up for AoPS. He takes the class after school and is allowed to do the homework on a computer at school. The only bummer is the teacher seems to need some kind of written record of math, so he also makes my son take math tests with the rest of the kids. It's so easy, it's not an issue, but it's pretty silly, IMO, and he always gets 100 percent. Next year, he's going to a science and tech magnet school, and they're putting him in the honors algebra class, but they said AoPS is more rigorous, and he'll probably need/want to go back to that. (Love that the school admits that!)
