This is purely anecdotal, but it seems like kids are held back in the US primarily by the curriculum. I immigrated to the United States having grown up in a couple of commonwealth countries and I was shocked to go from a school where I had been learning basic trigonometry to a school where basic algebraic principles weren't even taught until the next year. So whenever I see a discussion about all the social issues th…
Part of that is heterogeneous grouping. The curriculum is based on what the bottom 10% can handle, and everyone else must go at their pace lest there be elitism.
There's probably other factors too, though. All students could probably move faster if culture as a whole were encouraging it.