"Some children are more athletic than others, and some children are more intelligent than others. Starting among conservatives, but now spreading to some liberals, is a rejection of this premise via blaming teachers. "

That some people will be naturally better than others does not mean there are no low hanging fruit that could make people on average much more athletic and/or more intelligent. He doesn't explicitly claim otherwise but just to spell it out: that humans are not identical does not mean they are reaching anywhere near their potential. Teachers obviously aren't wholly to blame for any failings but the following possibilites are perfectly compatible: that there is variance among humans, that children are falling far short of their potential, that this is the fault of teachers (or the fault of the people that hire them depending on how you want to look at it.