DD9 skipped from gr 3 to grade 6 last year, after having done K and 1 in a year when she first started school.
The biggest challenge was convincing the teachers and administration that it would work and was appropriate for her. The problem we had was that it was really not enough of a skip - she still needs to put in limited amounts of effort, although the time it takes her to complete her work has increased, the intellectual input doesn't seem to be much greater. However, she has made friends in her gifted English class who seem to be pretty close to her intellectually and this has been the best educational situation we have found so far for her.
I would recommend talking to any teachers involved individually about your child's characteristics and let them have a chance to talk to your child before the school year starts so they can have a chance to get used to the idea of having a significantly younger child in their class than they may have ever taught before. (One of DD's teachers this year was very concerned because she had never taught anyone younger than 12 before and didn't know what she would do with someone so young. This teacher now sings praises of DD and wishes she had a class full of kids like her.)
I would also check out what happens in terms of social situation at the school. In our case, it is a small K-12 school on 1 campus (it is a public school), and I teach there, so lots of the older kids know me and knew DD before she came as a student, at least by sight, so we have not had any problems with bullying or being ostracized by others. In fact, there are a few kids who go out of their way to be sure that DD is hanging out with the "right type of kid".
