> By horrible coincidence this eliminates more cough lower performing children

If it's not scored it can't possibly eliminate low-performing children on that unconflated characteristic alone - a motivated underperformer will still get in.

It eliminates the unmotivated, which correlates obviously with underperforming. While it can be a vicious circle, I'd say no-motivation -> underperformance is of much greater relevance than underperformance -> no-motivation.

The obvious hint is how it tests the parents too. sure. maybe they are very motivated but just work so much they cannot volunteer or spare any time, but doesn't that also somewhat render their 'motivation' moot as well?