You're probably referring to the Ecole Polytechnique. Indeed newly graduates are supposed to work as a civil servant or pay the "pantoufle", which according to [1] is roughly 45000€, and as of last May when the article was written, it says only 10% of the graduates were paying it, but the government was starting to go after them.
I was indeed referring to Ecole Polytechnique, but also Ecole Normale Superieure. I think alumni from Polytechnique can be expected to join the army if needed, since it depends from the Ministry of Defense.
[1]In French: http://www.lepoint.fr/societe/polytechnique-le-gouvernement-...