Welcome.
My son is in a mixed 2nd and 3rd grade classroom doing advanced work. I don't think it's been an issue for anyone.
It may depend on how the classes are structured-- would a highly advanced 2nd grader work with 3rd graders or on their own? In my son's case, he has an individual curriculum for math- and occasionally participates with his class on general topic areas like measurement or graphs. It all occurs in a fairly sensitive manner and we've taught our DS to be humble.
Also, keep in mind, school success (at least in the eyes of other students) is about a lot of things... My DS is reminded of his struggles and his classmates' talents every time the class goes to PE.
