Do you mean clustered or differentiated?
To me differentiated is an individual focused thing.
Based on my DS, I'd say pre-algebra in first grade. Students in his GT class get instruction or material at their level. The talent coordinator did math pullouts in K, where ds was given stuff at his challenge level.
Now clustered, I would be looking at both style and ability. Maybe the high end group has a curriculum centered around 1.5 years growth per year, but further splits between the kids who gobble up big concepts and those who are more tactical in their learning approach. The big concept group would get pre-algebra exposure early like late first to incite their imagination and goal seeking learning. The 1.5 pace puts mainstream pre-algebra into 4th grade and of course has less spiraling and repitition.
