Afterschooling academics may result in rapid advancement well beyond grade-level curriculum, which is not, of course, a problem in and of itself(some may consider it a benefit), but does make it increasingly difficult to coordinate with conventional classrooms, especially without grade-skipping.
There are pros and cons to grade-skipping. Naturally, I'm sure you have good reasons for leaving grade-skipping off the table at the moment, but if you haven't yet done so, I would encourage you to review some of the research on grade acceleration, conveniently curated elsewhere on the Davidson site.
