1) Countries don't do an even job testing their students. In the US, everyone gets tested, even kids with severe emotional disabilities (meaning from broken homes and such). In some countries, only kids who are in the "college track" schools get tested. Yes, in some places young kids are tracked like that. In Germany students go to the Gymnasium, Hauptschule, or Realschulabschluss depending on ability. The Gymnasium is for kids who are going to university, the Realschulabschluss is for kids going directly in to the work force. Unless they changed it since last I checked, they only test kids in the Gymnasium with these higher level math tests.
So to retort the fallacy, why don't they test kids that dropped out after junior high with those higher level math tests?
"Ahead warp factor 1" - Captain Kirk