Still, thinking of it as inconceivable suggests we should simply give up on educating children of different abilities in the same classroom, and I don't think we should. That said, some kids do need to be skipped or put in different environments.
Even if there is ability grouping, so that 60 students are split by ability among 3 teachers for math and language arts, there will be variations in ability in each 20-student class, so "differentiation" will be needed. But deliberately maximizing the variation within each class through heterogeneous grouping is an ideological decision.
Every year our elementary school principal emails us about how she is assembling "balanced" classes for next year. Her definition of balanced is that each class should have the same number of dull, average, and bright children (although she would never put it that way). It is interesting when credentialed and reasonably smart people operate in precisely the opposite manner that you think they ought to.
