We are current K12 users, although we have chosen the Private School route with K12 International Academy. Our DD enrolled in iCademy at just 4yo and was placed in K with all 1st grade coursework. By the semester's end, she was officially placed in 1st grade and finishing up her 1st grade coursework. At the end of our school year, she had been accelerated to all 3rd grade coursework, and will be offically in 2nd grade doing said 3rd grade coursework. (all of this and she just turned 5). The iCademy has been very flexible with subject acceleration and grade acceleration. I feel very fortunate that we have not encountered any friction in regards to doing away with the busy work and in keep boredom and frustration at bay. Our teacher has been very proactive in ensuring that DD be placed at the right level so that actual learning occurs.
We are using EPGY Math in addition or in place of the K12 math, since DD loves the pace of EPGY. We just have to have DD take the end of unit Math assesments for K12 to show mastery of objectives, and that fullfills their requirement.