FWIW (although I have no research), I just wanted to add wrt the 6th grade skills. Supposedly the 6th grade class DD was in this year covered half of 7th grade as well. She had no issues despite being in a normal 5th grade math class with a variety of level kids (from GT 5th graders to normal).
I will say that in the 4th grade class (where she was a 3rd grader) she was not with GT kids for math. In the 5th grade class, she was with the GT kids who were a year older and there was an adjustment period wrt confidence. She quickly adjusted by the end of the 1st 9 weeks. This year in 6th she has to be close to the top of the class based on her grades. Just personal experience to show that they adjust to the "gaps" that may have been created by a different class mix from the year before.
