Here is my take. I am rising Sophomore in college, I go to Syracuse University.
I spent all four years in High School, my GPA was decent, 3.7.-3.8, my advice to you is spend the four years in High School. Especially if you have a lot of good friends, spending senior year together is awesome. We had a really tight nit group my senior year, there was 6-8 of us and we would all go to my friends house and watch cartoons during lunch. (we could leave school for lunch) I cannot describe how awesome our Senior year and Summer was, we had the time of our lives. If you have some close friends don't graduate early, enjoy it while you can. Also I have seen a couple of kids graduate high school early, and do terrible in college. Take lots of AP courses if you are smart, its a great way to prepare. There is nothing worse then going to college unprepared, it happened to one of my friends and now he is transferring.
Having 15-30 credits from AP classes is awesome when going to college, since u can skip the boring freshman classes and dive right into your area of interest.
Another big one is ECs. I got into John's Hopkins despite my GPA and SATs being well below their average, I think what Hopkins liked was the diversity of my ECs. I was the swim team manager for 3 years, class president one year, Student Council member for two years, Student Council Vice President my senior year, Robotics team member for four years, Model UN member for 3 years and being club president, I went to Boys State, and took third prize in a statewide film contest. Also I had my best college interview ever with the Hopkins admission staff. I was proud that I got into Hopkins, but I went to Syracuse because they have a great history program, as well as the best communications school in the North East, and they even threw in some nice scholarship money.
Stay in school and do the four years!