When I was seven, I was allowed to accelerate to our junior high's gifted reading program. This was the only part of my week where I was not bullied, beat-up, or bored to tears. In fact, many of these students welcomed me, and it was devastating for me when a new administrator reversed the acceleration and had me stay in an age-based classroom.
That being said, I think that a welcoming teacher and prepared classmates are essential for radical acceleration in the younger years (less so in later years--no problem even with unprepared college students when I would take classes or attend talks as a young teen/preteen). My teacher had allowed me to sneak into his classroom before I was transferred officially, and I also had friends in his class prior to the transfer.
