DS is going into third grade and last year in second they were sending him to 5th grade to work in an advanced group (6th grade math). DS is definitely not "mature" so I had the same concerns--and that the kids would treat him like some sort of freak, but that hasn't been the case at all. He has dyspraxia so it's not uncommon for him to flap his arms when he's excited, and the pitch of his voice will go way up. He actually has "social skills" in his IEP so awkward even compared to other kids in his grade. The curriculum specialist explained the situation to the 5th graders before DS got there, and told them that he's little but has a talent in math, just like they each have talents (like in sports or whatever the case may be), and he has a right to be there. The other kids are very nice to him. Today I asked DS if he's looking forward to starting school again and he said the only thing he's looking forward to is math. He said he could sit there in math all day. The rest of school he could do without. Feel free to print this and give it to the teacher (only partially kidding!). So much depends on the attitude of the staff and whether they want to make it work out just like you do.
The only "problem" that we've had so far with accelerating DS so much is that he is so excited to be there in that class, that he gets off task socializing with the other kids rather than working on math--it's something to work on, but it doesn't make or break the situation. The school is flexible enough that at some point, if the acceleration isn't working out, they can just put him into a different group.
