I could say that there was very little I learned from formal education until the 10th grade. I happen to learn some new concepts in Chemistry Honors and Algebra Honors then, but until that point little else.
The vast majority of what I know and have learned is from self-taught knowledge from reading from books and the internet.
My so-far limited formal education(I'm a senior in high school) itself has taught me some things about biology and chemistry and medicine that I might not have learned purely of my own motivation, but in the case of chemistry, I learned most of that by reading the textbook and then doing other things in class and ignoring the teacher. In the case of my biology class, I wasn't motivated enough to read the textbook(or to take it to class or home, for that matter), so I took notes and did other things in class and learned it. In the case of anatomy and physiology(the source of aforementioned medical knowledge), we don't have textbooks, so as in biology, I take the notes and otherwise don't pay attention in class but still learn it.
Calculus is something I did not from my formal education; however, I was taking the course when I learned it. I had wanted to learn it for a while(my first attempt was at around 8 years old, but I saw the strange notation and didn't realize the text explained it), but I had yet to actually do so. In part, this was because I didn't have any books on the subject and the wikipedia article didn't seem like a very good introduction. When I received my textbook for the course, I learned differential calculus. After a while, I got bored and learned integral calculus. I got bored again and learned how to do iterated integrals and partial differentiation. I'm glad I got bored so often during that year because the course itself only got us through definite integrals.
In American secondary education, I think that the main educational utility is from the provision of textbooks to the students. That has been, at least, the main utility of my school to me(other than exposure to such things as the creative writing and theatre clubs). My school is, however, a particularly bad public school in a very poor small rural town. It's possible that other formal education may be much better(I certainly hope that the college I attend next fall will utterly change my mind about formal education).
I think more to the point for many people would be: what percentage of the time you've spent in school has actually contributed to your repertoire of knowledge?
I think a lot of people might answer something like 20% for that, too.
Now that I'm back in college, this time assistant-teaching as well as taking grad-level classes and sneaking in time for research, I am more amazed than ever by how wasteful the academic world is. They chop up your day into many context-switches (one of the worst things a knowledge-worker can do), they focus on lectures and exams instead of mentoring and experiences that bring about learning, they require useless courses, they emphasize grades relentlessly (known to decrease learning), and they present material out of context (so it's hard to retain or apply).
I think good teaching can be done in the academic world (I have seen some), and of course it's the place to go to do scientific research. But overall, the academic world is just not very good at what it does (except arguing about minutiae and winning research grants and endowments). The basis of its strong market position is subsidies and prestige. It's ripe for competition from savvy entrepreneurs.
...and just finished last week. I'll be honest: if you really learned most of the depth in your field in college, and you've been out for a while, you've been a slouch.
What part of your knowledge comes from formal education?
In my case its at most 20%.
Edit: typo