Given that you have already had one skip with this school, would they instead consider administering an end-of-year test for each subject? I know that's not quite what you asked, but perhaps it is an alternative. It's what the school did for our 8-year-old a few years ago. He's accelerated two years now in math and language arts, although still with his "age mates" (entering fourth) for the music, science, art, lunch and recess.