High school is not mandatory. If you want, you can do self study to prepare for college, or if you have a job. Remember they have 3 years of middle school so when they go into HS they are 15 and usually turn 16 that year.
In France IIRC this is is not mandatory either. My dad did not finish HS due to some poverty (or Sicilian self-serving misery way of life to be exact) but got a diploma much later on.
But society don't exactly accept people that don't finish HS, they are seen as absolute morrons even if circumstances force them to get a job instead. I wish I could have just skipped HS looking back to get right into IT school, I've wasted 3 years for nothing in HS since none of that advanced math stuff helped me in any way as a programmer and youtube easily makes up for my curiosity in History, Science and Geography.
Same goes for college, before I got to be in a good IT school it was another 2 years of useless HS-level crap. Now that I've got a job I feel like "about time", but I still miss the 5 years of my life I've wasted over mostly thin air and lots of coffee.