With the consensus already in place, I think that a one-year acceleration to 5th grade would not be particularly risky. However, I would be uncomfortable with a two-year acceleration to 6th grade unless your 6th grade is still consider elementary school with all that scaffolding/hand-holding still in place and/or your DS9 has highly advanced executive function and a decent physical size. As high ability as my DS11 may be, he could not have handled the executive functioning demands or the huge 4-inch binder as a nine year old.
