The most useful thing I learned in school was the Multiplication Tables. There's never been a day in my life when I didn't use them. I hated "Pop" Corner for his regular mental arithmatic torture sessions, but I'm grateful for the results. (I'll never forgive him for all those thrashings though). Differential Calculus was quite useful too, but I don't use it much any more.
The second most useful thing I learned in school was that girls are much more interesting than boys.