I personally would be very hesitant to put a kid who was truly struggling to produce grade-level material with similar-aged kids working two grades up. Particularly in a class with two similar-aged kids working two grades up - in a group of 3, you have a high likelihood that two of the kids will bond and exclude the third, and "we're working on the same classwork" is a huge bond.
My DD9 is probably capable of producing 6th grade classwork in a one-on-one tutoring situation, and would probably love it. I'd be hesitant to put her in that school, just because the dynamic of 3 is so bad.
