At low grade (K, 1, 2), it may only require CBE (Credit by exam) on Math and English.
At higher grade elementary, it may require Math, English/Language, Social science, and Science. You'll typically have to score A on all of them to be allowed to skip grade, which is doable, but certainly not easy for most kids.
Of course, you want to make sure the kid is mature enough to be social with peers that are older. Better yet, will still be in a position to lead in his/her peer group even being the youngest.