I'm also a primary teacher and would say that if he is turning 11 in April that would make him a VERY young grade 6 student. Remember they go to secondary school in year 7 here so he would be young all the way through. Also because boys are "generally" less mature it might benefit him doing grade 5 in Vic. It wouldn't have to be like repeating if you explained it right. Also he might be okay now in grade 6 but would he cope being that young when he is in year 11 or 12? It's easier to start grade 5 now than to repeat year 12. It would also depend on how bright, organised,mature etc. your son is. Just a few thoughts!