t sounds so democratic, a very American idea: break down the walls of “remedial,” “average” and “advanced” classes so that all students in each grade can learn together, with lessons that teachers “differentiate” to challenge each individual. Proponents of this approach often stress that it benefits average and lagging students, but a new study from the Thomas B. Fordham Institute suggests that the upsides may come at a cost to top students — and to the international competitiveness of the United States.