Wow - that sounds like a fabulous school! I wish we could send our HG+ DS5 there too. I don't have any advice yet, as we are in sort of the same position. Our DS5 will be starting kindergarten in our local public school. The school doesn't have any official gifted program until 3rd grade, but they recognize that DS should not have to sit through basic learning to read and beginner math stuff, so they're going to provide some differentiation. But I'm green with envy hearing about your school with a planetarium and real science labs!
I have read that some HG kids really like kindergarten, since it's not all about sitting in your seat all day long, so hopefully this will be the case with your DS. And perhaps the nonskipping policy could be readdressed when the school sees what your DS is capable of.
Sorry I'm of no help, but I'm also interested in hearing what others have to say with regard to your question.