Is it just reading and discussion groups? If just that, I can think it can only be positive to discuss books that your DS might be more interested in. Is there a writing component? If so, how is your DS with writing? I ask because my DS-almost-7 refused to learn how to write until he went to kindergarten, and he skipped first, and his writing is still pretty bad compared with the 2nd graders.
I wouldn't worry too much about what the 2nd graders think. At that age, kids don't really seem to care too much. As for pullouts, my DS was pulled out quite a bit in kindy - for speech, math, and GT -- and at the time he didn't like getting pulled out so much, because he stood out. But now, a year later, he tells me he likes his pullouts (still GT, math, speech) because he sometimes gets out of some things he doesn't like.
He's no longer worried about standing out, anyway! And for kids who are advanced, they likely aren't missing too much in kindergarten.
