This method,or something very similar,has been a godsend for our son. He goes to a central school (public) which are all K-12.He was offered grade skipping but did not want to go down that path, preferring subject acceleration instead.
They also use the method of subject grading of kids according to ability.Subjects timetabled concurrently are english,maths and science extending over Yrs 5-8.He is now 16 and in year 10,doing year 11 physics and is also enrolled at university in a Bachelor of Science majoring in Mathematics and Applied Physics via distance education(he was accelerated 3 years in maths).
I don't know how the school system works in the US and I am new to this forum, but this pathway isn't very common here either and that's probably understatement!
Cheers,
matmum