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?