I was handed math textbooks a few years above my grade when I was in 5th grade, and it kept me interested longer. I think I overall preferred that, to being moved. Socially I was definitely not ready for being moved into higher grades etc. In highschool my maths teachers mostly let me do my own thing, other than asking if I'd be interested in helping others on occasion, and frankly, that was probably more valuable to me than advancing faster in the subjects. Not trying to contradict you, btw., just another option.