If you use K12 independently, you can place your child anywhere you want. If you use a virtual academy, you have to go by their rules. I believe that WAVA allows the child to test into the initial levels of math and language arts. At least that's what they did when I looked into it--the child would be placed at his/her level in math and LA, and at grade-by-age level in everything else.
K12 has its good points, but some of those good points only apply to using it independently. As an independent user, I can place my child anywhere in the curriculum and go at whatever pace I want. It's a rich curriculum when compared to other "regular" school curriculums, but it can be deadly boring and require too much busywork (defined as output that doesn't further learning in the subject being taught). As an independent user, I can eliminate what I find to be busywork, but I find that there is sometimes so much to eliminate (especially in language arts) that it gets to be ridiculous.
I have used K12 for my just turned 8yo since he was 5. He has done their Science 1-4, Language Arts 2-4 (parts only), and History K. I have also used it with my 13yo. He has taken Life Science (the "advanced" option wasn't available at the time), Literature B, and Literary Analysis and Composition (just the literature piece for both of those).
We have also used their world history series The Human Odyssey, and it is the best mid level history spine that I've ever encountered.
We have never done the math.
