Ah the Technische Hochschule!
I lived in the German community for 4 years on my last tour. We had various neighbors within shouting distance whose kids were going to vastly different schools and some had been released to go altogether to trades.
This is the best explanation why our 10 year olds score the same or better than their international counterparts. We're surveying the entire population at that point.
This is just my opinion, but it's based on my experience. Incidentally, I was told of a similar system when I was stationed in S. Korea.