Our DS6 skipped K and was in 1st this year, and it's worked out just fine. He was in a half-day preschool for two years and went to full-day 1st grade. We're going ahead with 2nd next year, but with an eye toward skipping if it seems wise. He has frustration issues with the kids not doing things the way he thinks they should, and kids doing the things kids do regarding friendship and playtime--which actually makes it better to be with the slightly older kids, because there is a bit less of that and a bit more tolerance for his frustration. Academically, he is torn between wanting to do what he is capable of and interested in and not wanting to do something different from the rest of the class. I would have to say he adjusted well to the longer day and the school routines, and it was good to at least have him in a classroom where he had higher-level stuff available to him--preschool would have been hard-put to find anything useful at all for him to do. We have had our ups and downs, but if we could have done anything at all to solve the problems this year, it would have been to skip him further ahead, not move him back (even though he occasionally said he wanted to go back to preschool).
Good luck!
