DDjust12 was in a slew of classes with older kids, in some cases significantly older. We explained to her that being in these classes was a privilege and that she had to act with appropriate maturity (and lets be honest, sometimes the 14-16 year olds are just as immature as little kids). I checked in with her teachers and found that she did extremely well.
The two issues we had were:
1. Being out'ed by classmates who then treated her a bit differently (you're only 11, we thought you were 13!). Your DS is going to probably be different enough in appearance for this to be obvious from the start, which might actually be better.
2. Being more immature outside of class. With her closer-in-age friends and at home (who are all around 13) she's more immature than you'd expect. While we encourage appropriate behavior, we do see this as a bit of a coping mechanism... like letting off steam. Your DS will have his social time with his friends too, so this is good.
