Regarding the K12 curriculum, we used it for math only in 4th and 5th grade. DS was at his bricks and mortar public school for everything else, but he took math through our virtual academy.
We found it quite flexible. He took the placement test before he started and passed out of 4th grade math. He started in 5th grade math book and moved through pre-algebra B by the end of 5th. I typically gave him the end of the unit assessment at the beginning of each unit to see what he knew. Then we only did the parts of the unit that he needed help with. I had to go online periodically to document what he had passed and to document how many hours of math he had done.
In return, I have the official transcript showing he had finished the pre-algebra sequence so when he went to middle school this year, they just placed him in algebra without any fuss.
I liked the elementary curriculum they used; there were lots of projects and hands on activities. We thought the pre-algebra sequence was really dull, but because we could skip a lot and I could add stuff to make it more interesting it was OK.
