It is not only owed to the Ecole Normale Superieure where the 11 Fields medalists were educated, but the prominent “Math Culture” in French society.
This is similar to the International Math Olympiad (IMO) “craze” in China since 1980s till today, where the parents send their primary school kids to drill in IMO boot- camps, because that is a “direct-entry” gateway to enter top university, bypassing the highly competitive “Gao-kao” 高考 (University Entrance Exams for 500,000 places among 7 million students each year, only 7% successful chance !! vs Singapore 40%).
Quora: read how this middle-age (48) French recounts his “French Math” education since 6 to 15 years old:
The curriculum was designed in the late 60s in part by a group of real mathematicians, the Bourbaki. It was very abstract. I learned about basic set theory when I was 6. I did learn about the basic operators but not before I was able to perform them in arbitrary bases from 2 to 10. I was taught the properties of ordering relationship, equivalence classes and partitions at age 11, vector calculus at age 13. Basic differential calculus was taught at age 15, complex numbers and integral calculus at 17. This was carried over to physics where we were happily solving ODEs for mechanics and electrical circuits at age 16. This was for almost everyone except the most dyed-in-the-wool literary person. I was taught elementary proof techniques at age 13. By age 15 I knew about formal logic, contradictions and recursive proof.