A High School is a general purpose educational facility, usually for those between the ages of 13 and 18. It provides foundational knowledge, such as instruction in literature, mathematics, and scientific concepts. Some of them also teach life skills, which vary according to when you live. Some schools where I'm from teach shooting, for example, but that wasn't in the curriculum pre-Rising.
You can't always tell which knowledge will be useful. Things change. An understanding of zombies was considered superflous when my grandparents' generation was growing up. There are things that will be useful in the future we don't know about yet. Besides, learning conditions the brain.