I knew from the beginning school was preparation for adulthood, but it took me awhile to figure out exactly how. Here are some of the useful things I learned in school: show up on time, follow instructions, keep track of important papers, fill out forms, communicate clearly, and other skills directly useful in daily life. All that stuff about vocabulary and geometry was just there to keep my brain busy, not because I was going to need it later. Even so, keeping your brain busy (with anything) is a good habit.
Of course, learning can also be fun, and the word school itself is garbled Greek for leisure or free time. In the ancient world, only rich people were free to study things like poetry and music. Everyone else was too busy learning and doing practical work to study. In a similar way, reading for pleasure only became common when advances in technology made books cheap enough for average people to afford. The free time most people enjoy today was also made possible through better technology, particularly mechanized agriculture.
You don't need to read a lot of books to be wise, but it certainly helps. The longer I live, the more I think that intelligence is less a list of things to know and more a set of habits to practice. Most knowledge is not particularly useful, but good habits always make life easier.
As far as social skills, I learned that people mainly get judged on how they look, what they sound like, and how much money they have. High academic achievement or intelligence doesn't get much respect. If I was a smarter man, I would have figured that out sooner. However, I don't regret my education because it's enriched my life in unexpected ways. That's the real lesson.