I'm going to beat the same old drum and say that many of these problems would be ameliorated by doing away with age based cohorts. Innately low ability children would still benefit from the presence of high ability students at all levels, with the added benefit that the high ability children would be motivated to achieve at their potential. The aspirational benchmark for non-GT students would be higher, and there would be greater commingling across abilities, presumably with trickle down.
The slides are pretty dire. Talk about a straw man presentation.
