My younger dd finally told her teacher that she wasn't going to waste any more of her time doing the pre-algebra book. At that point she was "pretesting" through the chapters...and dying of boredom.
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For a really mathy kid, I'd skip it.
I had a terrible teacher for pre-algebra. She was the worst I ever had by a large margin. I totally tuned out.... the next year I was the top student in algebra.
I agree that mathy kids can skip it with no problem.
Actually, I think half of 6th grade math was review, because the elementary school children had been pooled into the middle school. There's about 1.5 years of math there that were a waste of my time.