Pretty much like an adult, unless you're too young to feel safe on the roads...

EDIT: Some more advice. Go for a rides before school on nice days if you have a free block. Ride your bike to school if you can, it's a good way to keep the legs snappy if you're too busy to ride after school. As I said, any adult training program will basically work for a junior if you have time and the means. Pick up a training book, such as The Cyclists Training Bible by Joe Friel, although I find some of the material daunting for someone who does not, or is new to, HR/Power training.

The best advice I can give is remember why you started to ride - because you love it. Put any delusions of grandeur in the back of your mind, and just go ride.

By the way, I am a junior.