Well, you could have your DH (assuming you are married) or a friend call to ask the school about the policy. However, unless you know of kids who have been grade accelerated, I think you can probably assume they're not wildly in favor of grade acceleration and may be actively opposed/hostile to it.
But the fact is, even if there's a policy in place, schools will use grade acceleration if it's the easiest solution (for them) to a child's advanced position in school. They'll fight against it tooth and nail, of course, but they'll do it if they have to. The issue then becomes how to make them have to...
In those cases, it usually takes a VERY squeaky wheel, though. Lots and LOTS of very patient but persistent advocacy. (Personally, I didn't have it in me! Many parents do, though, and wind up with good results.)
If you can't find out the grade acceleration policy easily, without raising red flags, then I'd counsel you to assume you have an uphill battle on your hands. If you're wrong, then the process would be easier than you expect. If you're right, you're prepared. No loss either way.
Reading with interest...
