To give you another perspective, our DS is also a (young) second grader and is now in fourth grade for math only. It's working well. The teacher was supportive but a bit concerned about how he would fare given the maturity differences, etc. She's fully supportive after about a month of this acceleration. He enjoys the class, although he still gets frustrated with some of the methods (lots of drawing out diagrams to show how the numbers work, etc.) when he knows the answers in his head or through traditional steps. According to the teacher (confirmed by DS, though not in as much detail), one of the girls watches out for him and he gets along very well with the other students. He's helped some of them with some concepts and they think that's cool.
On the other hand, at the start of the year, he was in third grade math with a less supportive teacher and wasn't nearly as happy about being in that class. (Liked the kids, complained a lot about the work/homework.)
