Bruce Farrer is the man behind this particular assignment and he's been doing it for decades. I remember doing something similar in highschool, but it was only a single page letter and we opened it back up again 4 years later in our senior year. Farrer, however, makes his students write a 10 page letter to themselves and faithfully tracks down each student to deliver their letters 20 years later!
Imagine receiving a letter from yourself 20 years ago and how different you would sound. Would your dreams from 20 years ago be realized? Have your dreams and hopes changed? There need to be more teachers like Farrer.