To get the European perspective, at least the UK one from many years back!! At 13 I had to pick subjects, that was all that was taught to us. I picked Math (compulsory), English Lang (compulsory), English Lit, French, Chemistry, History, Geography and Art. You did those subjects until 16, which is the age you can leave school at. You took exams in each subject. If you wished to go onto college you had to do a further two years of schooling, were you usually picked 3 or 4 subjects (I picked, Art English and Business Studies) Again you had exams, your grades in these , and also your grades form the previous exams were taken into consideration when applying to colleges at 18 years old. If you had no Science subject, there was no way they were going to let you do a Biology Major degree. With degrees, again you were expected to pick a subject on application (I picked Law) All the way you were being asked to narrow down your choice.
I completely agree with the idea that how on earth do you know what you want to do when you finally leave school, and plan your subjects accordingly. One of the worse things was that sometimes you had no choice in one of your subjects as that was the only thing that didn't clash (class time wise) with the rest of your picks. It did vary from county to county, even school to school. My first High School had a much broader approach. I was not a happy camper when my Mum moved, as I would have a much better range of qualifications if I had stayed at the first school.
I know it has changed, some college degrees are now more flexible and more in the US modular model. Schools though still expect you to pick examination subjects.