My DS6 is rushing through easy work and producing sloppy results. Specifically, he is being given color by number (literally 1, 2, 3) or color by word (pink, blue, red) worksheets and getting written feedback regarding the neatness of his work. He is getting the "facts" right on these worksheets but the only feedback from the teacher is "PLEASE SLOW DOWN AND COLOR NICELY".
At many elementary schools, including those of my children, unfortunately, there is heterogeneous grouping, no grade or subject acceleration, and no gifted program. Age determines all. This is a bad system, but perhaps one saving grace is that you have little to lose by being mildly noncompliant. So if your son's school is like ours, he can color quickly, use the extra time to read a book (I hope), and perhaps accept a slightly lower grades in 1st grade. If your school had a gifted program that depended on teacher nominations, the calculation could be different.