Health class in 9th grade. The contents of the class was this:
Smoking will kill you. Drinking alcohol will kill you. Premarital sex will give you VD and ruin your life.
At the age of 14, I didn't smoke, I didn't drink, and I desperately wanted to have premarital sex and didn't care about any consequences.
What's one class in school that you thought was pointless at the time, but later realized was important?