We can imagine that social mobility might be one of the things that you think about concerning a school system. But to set it as the one and only over-riding measure seems to us to be going very much too far:
Heads are being urged to "vigorously" oppose the expansion of grammar schools in England.
The National Association of Head Teachers will debate calls to reject a new wave of grammars in the absence of evidence that it aids social mobility.
If grammars can only exist if they aid social mobility then that is indeed insisting that social mobility is the only measure of the design of the education system. Which does look like, to us, an error.
It's entirely true that there's not a great deal of social mobility around. But as Greg Clark has shown, that's also true in places like Sweden that don't have grammars. And there's also that long insisted upon point that grammars increase social mobility by providing the bright from any background with an academic education.
But we would still insist that the basic contention here is wrong. It cannot be true that social mobility or its furtherance is the only measure of a school system. We really do think that we want to consider other matters too, like the costs of the system, what it's teaching perhaps, possibly even whether the system is teaching anyone anything at all. That last being something we're really not all that sure about for at least some British schools.