Ah, the typical French delusion that entrance exams make Grandes Ecoles admission somewhat fair.
The fact is that most of the students who succeed at these entrance exams and land position in the most successful of Grandes Ecoles come from the same highschools. These highschools all happen to be in Paris or Lyon and due to how the French system works, your family has to live in one of these very affluent and costly neighborhood for you to attend. Amusingly, practices which are mercilessly squashed by the administration in most highschools throughout the country like skill-based sorting of students or going at a faster pace that the official curriculum are tolerated in these few highschools. I let guessing where our elected politicians put their children as an exercise for the reader.
As usual in France, there is one rule for the plebeian and another one for the ruling class.