That's not unreasonable if you want to select for high performing children. The correlation is very clear.

There are various correlations which are very clear, like parents wealth. The question is whether these are ethical. If every school filters for such criterion, would students from poor or uneducated parents ever have a chance at social mobility? Do we want such a society?

In the US, the voting majority seems to want such a society, otherwise we wouldn't have so many politicians cutting education and other taxpayer funded support to families in need.