This is a big problem with our schools, as well. Each school houses a small number of grades and the schools are NOT located near one another. Right now, I have one DYS working on above-grade material in one subject in the regular grade (most of the time by himself, without a dedicated teacher), because I suspect that it is simply a logistical nightmare to figure out how to get him to the correct grade building. Next year, he will probably just be allowed to walk to the next grade for this subject because he will be in the next level building. In the extraordinarily rare occasion that subject acceleration has been allowed, it is my understanding that the parent agreed to transport the child. We may face this issue in the future. Honestly, arranging for transportation would be far less expensive than changing schools, so we'd be willing to do it in order to make it work.
Meanwhile, we have family in a neighboring town with a similar issue. Their children were transported at district cost to and from the next building for one subject a day.