It works beautifully at my son's small private school, but only because class size is around 8-9 kids, and teachers are given the time to develop an individualized curriculum for each student. After a week of review at the beginning of each school year, all kids are pretested in core subjects to determine their readiness levels. Then, the school literally closes for a week so that teachers can prepare individualized plans based on the pretest results. In addition, throughout the year, students take a pretest at the beginning of each new chapter. If they score 80% or higher, they get to skip that chapter and move to the next one. So, it is rare that even two kids in the class are working on the same math or language arts assignment on any given day. The teacher floats around the students and provides one-on-one instruction as needed. However, for "specials" like science, art, music, Spanish, drama, or PE there is no differentiation, and all the kids work at more or less the same pace.
In this system, my DS7 is working his way through a 4th grade math book and 6th grade spelling. Other kids in his class are doing working at, below or above grade level, as needed. And the best part is, my son thinks this is all perfectly normal.
