2nd grade for DS: The teacher always trained student teachers - she'd put the student in charge, and then pull kids back one at a time to work with them on strengths or weaknesses. DS got taught 2-3 missing 3rd grade math skills then.
Our school has a 4/5 compacted math for kids who test gifted in math. It covers the two years in one, plus additional problem solving skills. The next year covers just 6th grade math, but tons of extras parallel to a typical math curriculum. This year, 5th graders who tested highly for the first time are getting 5/6 compacted as well.
