Some people just like to be challenged and the OP sounds like that type of person. He's finishing high school at 16 and he's taking AP courses. Why waste your time in a full semester when you can just spend one month studying to pass out the course? If you are able enough to pass a college placement exam and you never had any formal training in it, I wouldn't call it flimsy knowledge. I'd say that your IQ is higher then the general college population and your time is better spent elsewhere.
Also, some Universities offer Mini semesters that last for 3 or 4 weeks for certain courses. They are generally offered between regular semesters.