First, social position comes not only from economic capital, but also from cultural capital (mastering of language, culture, knowledge of social codes, social skills, diplomas...), and social capital (social links, networks...) as well as symbolic capital which is institutional recognition. Teachers' (and doctors') children, even when they are not economically rich, usually have a much higher cultural capital as well as a stronger social capital than workers and low-level employees. Also, teacher and doctor are symbolically more valued in the collective representations of the social system.
Second, for teachers, there is an insider's advantage: they know quite well the functioning of the national educational system. Combined with their specific social capital (networks), this makes them able to help their children to find the best ways within the educational system.
That doesn't mean we have to blame them for that: they do their best as parents to help their children. But it explains why teachers' children are overrepresented at the top of the French educational system.
"Dieu se rit des hommes qui se plaignent des conséquences alors qu'ils en chérissent les causes" Jacques-Bénigne Bossuet
If culture is a good indicator of success, I would say that is a desirable situation. Far more desirable than if it is parents' wealth. For example, you would be unlikely to see people with no culture, like Bush or Sarkozy, rising to the top (my inclusion of Sarkozy is also to show that I am afraid that culture is not the top factor in determining success even in France).
As for getting culture, well, the French system forces you to study many subjects for long (unlike A-level where you can take as few as 3 subjects). So if a child is a cultural sponge, he will be exposed to it and will have chance to absorb it. I sure have seen people from very humble backgrounds with a desire to learn. When they are children, they are the ones Jérôme was talking about when he said that the system used to identify bright kids everywhere.
I, personally, find it a very good thing when teacher's children are overrepresented at the top (except for the insider's argument. I don't know how much of an impact it has, my mother never ever used any network, but one case does not a statistic make). It means that positions may be earned rather than bought. It means that people chosing a career where they must care for others may sometimes benefit from it. It meast that opening the minds of your children may pay off.
One should not forget how you get in a Grande Ecole. It's a competitive exam, and anonymous. You can't convince teachers to make your child look good in return for contribution to the school. You can't get in because your parents got in, and so on. No system is perfect, but that one is fairer than most. "Few can believe that suffering, especially by others, is in vain. - Galbraith"
Statistically, at birth, their is an equivalent number of "cultural sponges" or children "with a desire to learn" in every socio-economic category (unless you think the social structure is genetically determined). But children coming from workers or low-level employees backgrounds have much stronger cultural and social liabilities. "Dieu se rit des hommes qui se plaignent des conséquences alors qu'ils en chérissent les causes" Jacques-Bénigne Bossuet
