I have, in the past several years, taken 2 graduate-level courses on teaching methodology: one was a course on instructional design geared at corporate instructional designers; the other is a course geared at classroom teachers who teach reading. Both courses were offered by organizations that provide highly-regarded accreditation.
In both classes the issue of 80/20 came up... do you teach so that 80% of the class has mastery or that the last 20% has master? In other words, do you teach to the majority or the struggling minority?
In both classes this question was explicitly addressed by the instructor:
In the case of the corporate instructional design course we were instructed to teach to the struggling 20%.
In the case of the reading course we were instructed to continue drilling until 80% of the class had mastery.
I understand that there is no remedial instruction in corporate training. Training is training, when it is done everyone we trained is expected to perform the task they were trained on to a basic, pre-defined level. Schools have specialists and special ed to help the struggling 20%... but it still feels like there is something wrong with the attitude that it is OK to go on while 20% of your students still don't get what you have covered. But maybe that is the corporate instructional designer in me.