I think I explained myself poorly. Of course the schools don't measure raw IQ, there are many factors like intrinsic motivation (problematic as a measure in young children) conscientousness (which is a relatively stable character trait) and of course parental pressure which distorts any measurement we can try to make. All I was trying to say that it is possible to judge the potential of the children with adequate accuracy.

Your claim the IQ does not change after 11 years old, even if true, is completely irrelevant then. It shows neither that it is good idea to split kids as 11 years old nor that you can say potential so soon.