An important point to consider is that some kids work one grade above level because that is their actual challenge level while others are stuck at or slightly above grade level because that is the extent the schools will allow despite the fact that the student could progress much, much faster based on innate ability.

The lock-step issue causes much more angst for kids who are capable of completing 3 or more grades per year, but instead, are forced to spiral with the rest of the class year after excruciating year.