In general, I am opposed to it. There are numerous math concepts that could be explored in depth rather than just moving on to the next grade: combinatorics, discrete math, optimization problems, etc.
Open Middle and NRICH are two excellent resources for such problems.
I want to do all I can to prevent math from being something we race through. For students who need more, let them explore every beautiful nook and cranny of their own grade level before moving on.
All this being said I have had a few extreme cases such as a 3rd grader taking calculus and an 8th grader taking college courses.
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