It sounds, actually, like a good plan to me. My older dd's pre-algebra was extremely repetitive, fairly basic math. If you can get the syllabus for the class, your child might be able to skim the book to see what he/she doesn't know. The khan videos are also another place to fill in the blanks. As far as I know, pre-algebra classes aren't typically "tracked" so you may have children who aren't "getting it" slowing things down a bit
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. This worked in her montessori classroom, and she just went on to more challenging math. This was a few years ago, and it hasn't been a problem with her grades or understanding.
For a really mathy kid, I'd skip it.
