[yaantc]

You can find some guides on the topic, for example: https://www.letudiant.fr/etudes/classes-prepa.html.

Very often, when a kid is good enough its school main teacher will mention the possibility of going to a prépa: it's good for a school reputation when they can send kids to prépas.

For sure it's better when the parents are already in the known, but it wasn't the case for me and it was not a show stopper. There's plenty of public information and the importance of "grandes écoles" in France is not a secret.