So far, I've been totally unsuccessful in a similar situation with my own son. The school SAYS they're doing what they can, but in reality are sweeping him under the rug and keeping him around so they can report and take credit for his standardized test scores. So I'm not sure that I really know what I'm talking about here.
I do think, however, that the most important point in challenging a child is to focus on the future. I've always said that it's really nice to have a great report card come home, and for my son to not really have to try. But if he gets into the habit of doing minimal work at school, then when he gets to middle or high school where it's a little more challenging (not to mention where his grades actually matter) he will have no experience putting effort into anything, and fall apart. It is also a proven fact that students who are not challenged tend to lose interest over time, and often end up with grades that are even lower than the average student. I would bring these things up first, so they know that you are concerned with the welfare of your child, not just looking for a reason to brag to the other parents.
Ask them about how they plan to ensure that your child is challenged, and definitely bring up differentiation "just in case". They will respond better if you don't come across as someone who KNOWS their child is brighter than everyone else (unfair as it is, they will just think you're a snob and brush you off). If you have samples of above average work that he has done, bring it with you. Ask how this compares to what they will be doing in class. Then maybe THEY will bring up some sort of acceleration or something for him, which will make it a LOT easier to get something done.
Good luck, hope this helps!
