My DS was accelerated 3 grades for math, but it was never viewed by anyone as inflexible or written in stone. They put him where he needed to be based on his test scores with the thought that he could be moved somewhere else if it wasn't working. The difference though is probably that they have the math schedules aligned and a lot of kids do this so there isn't a lot of complicated schedule manipulating. Probably 15 percent of my daughter's 5th grade class went to 6th grade for math, for example. DS is unfocused as well and his written output is terrible because he has dysgraphia. But it would be the same way in the regular classroom so why not put him w/ the correct level material? I think when schools' get hung up on "output" rather than "learning" is when you the brick walls get put up.
