... I am breaking my break-off commitment for the purpose of this diary.
As a graduate of a French Grande Ecole, there are some points on which I would like to comment, at least on the top of this thread, and apologise in advance if these have been addressed lower on.

First thing : it is not accurate that you cannot get into a Grande Ecole without undergoing the 2 (most of time actually 3 years) of Classes préparatoires.
Grandes Ecoles have for several years had a quota of intake of students who have spent 2 years at the uni (DEUG level).
This BTW is often questioned by those Grandes Ecoles students who went through the traditionnal Prépa system.

Second thing : well done diary, as usually, but I would have two questions :
1. why are Centrale students  being left aside ? I may have lost track of the recent developments, but to my knowledge, Ecole Centrale still exists, with two sections (one being literature, the other science).
At the top of corporate ladder of many French companies, it is either the X clique, or the Centrale clique, each ruling the other out. The latter like to point out that they by far outstrip the X.

2. why are top business schools left aside ?
True they do have such hegemony in companies boards as the ENA-X cliques, however the merits of schools such as HEC, ESCP-EAP and ESSEC is that they train managers and people who learn to adapt themselves, and not people who are taught that there is only one truth, and one reality,  that of facts (X and Ecoles d'ingénieurs) or that change is dangerous (ENA).

I would add that business schools, with their policy of growing international, are better known and recognised in other European countries and in the US than Ecoles d'Ingénieurs who so far have been  more "frenchy".
ESCP has had a double-graduation programme with TU-Berlin and Oxford for many years now, and HEC is worlwide known for prestigious jvs with LSE, Wharton, etc.

When through hell, just keep going. W. Churchill