Having been married to a teacher of 31 years who teaches gifted children and teaches teachers differentiation, it's my opinion one of the largest issues in this problem is one that almost nobody ever discusses, the simple fact that teachers coming out of college have little or absolutely no training on the concept, let alone methodology, of differentiation. Even after a decade of teaching, one is lucky if they get half a dozen hours of in-service training. How good can we expect teachers to be at differentiation when it's not a topic the colleges and public schools think is worth training teachers in?