Our DS11 (he was 10 at the start of the school year) is twice-grade-skipped in 7th grade, and he was in 8th grade math for the first half of this year. He's never had any problem being with the older kids -- in fact, it's better with the older ones than the ones his own age.

For the second half of this year, he went into the second half of Algebra 1 in our e-school, and it didn't bother him a bit to miss the first half of Algebra 1. He's doing better (97% so far) in it than he was in the regular 8th-grade math class, because he doesn't have any problem remembering to turn stuff in online.

I'm not sure I would have bothered, for only one quarter, though.