Strictly speaking, Brainpop would probably not be considered a K-12 program. However, it can provide a flexible source of material, presented in an unusual and engaging format.
I initially learned about Brainpop because our (9 year-old) daughter was watching for fun. Now this was interesting, because Brainpop is a learning platform, and she was seeking out the videos because they are interesting and entertaining.
Eventually, I decided to purchase access to all the content, which also gave me access to the “Educators” section. Here, I found that I could create a curriculum based upon the area (state, province, or common core standards); subject; and grade level. The output is a list of Brainpop resources (videos, quizzes, games) which satisfy the criteria selected.
Or … I could create my own completely individualized curriculum.
As our daughter completes each (graded) activity, she can send me a completion email. This allows me to not only keep track of her activity, but also provide documentation for end of year assessments.
Another possibility is Khan Academy. Although KA groups subjects (particularly Math) by grade level, there is no requirement to stick to the listed order.