Some schools will allow you to have your child assessed and then enter the desired grade level on a probationary period. Be persistent. CIS (Canadian International School) in Khalifa has a cut off date which is 6 months later than most others, or at least it did when I looked into it 2 years ago.
We had this same issue as both our girls are young for their grade, with our State's cut off Dec 5th. I have children at Raha AND ACS and both are 'too young' for their class, as are several others in each class, so it is possible. For those in this situation with the American Community School, be very nice to the registrar at ACS and be firm with their counselor if he recommends grade retention. They insisted that one child was ok to not repeat a grade and the other would have to repeat her grade. GEMS and Raha did not agree-so I have kids split at two different schools. Make sure you can get strong letters and report cards from their current teachers, and just persistently smooth talk them to death.
Raha doesn't have the waiting list that ACS has so there is more flexibility. ACS can do what it wants, so play nice with them ;-). I think Raha, and their IB curriculum, would be a good possibility.