My DS8 took the k12 assessments and tested out of highschool, which he is certainly not ready to be. My opinion is that the tests are not deep in any way: there are only a few questions on each topic, and they are not timed. So, for example, my son was able to look at types of math he had never seen before and take as much time as he needed to just sort of figure them out--without really knowing the theory/properties behind the problems or how to show the work. He was also able to work out a lot of the reading questions simply because they were multiple choice. Could he write a paper about text structure? No. Could he guess which structure was used in a particular passage by using his reasoning skills to pick the best answer? Sure. In short (too late for that now!), I think the tests might point you in the direction you need to go, but I don't think they will tell you exactly which grade level your child will need.
As for the k12 curriculum, we are only about three weeks in to using the k12 curriculum through a virtual public school--and any complaints I have about it are based on that. The curriculum itself seems fine so far--especially if used as a starting point rather than an ending one.
I don't know what grades you are curious about, but my 8 year old is in 5th grade for most subjects and 8th grade for math. I also have a 5 year old in kindergarten. If you want to hear any specifics about those grades, let me know.
Good luck!
