I have only had time to skim the thread but I wanted to share our mid-year grade skip experience.
The one major problem my DS had was that the staff at school had not been made aware of his skip. In an effort to not make much of a "to-do" about it, the principal didn't tell anyone other than the receiving teacher.
Day 1 of the skip, my DS was in trouble at recess for being out with the wrong kids, in trouble at the lunch table for sitting with the wrong class and in trouble in the library for coming in with a different class. His teacher did a good job of shielding him from some of the questions but she couldn't catch all of it.
It took about a week for the message to spread so that people stopped questioning him, taking him back to the wrong class, telling him to go to the office or putting him in the wrong line at morning line up.
If you care, you'll want to make sure this stuff doesn't happen! We had practiced how to answer why he was in a new class "The principal said I'd passed the tests" but not what happened when people sent him back to the wrong class!