The local army recruiters hooked me up with a high school over in Michigan that had a correspondence program for adults who wanted to go back and earn a diploma. I would sign up for two classes at a time and leave with the textbooks and a thick folder of homework for each class. I would sit in my room and work through it 8-10 hours a day, usually finishing it all in about three days. Then I would go in and take all the exams that you would have to take if you were in that class for a whole semester.
I went from being a year behind in school to graduating and leaving for basic training a month before I was even legally old enough to be awarded a HS diploma in Michigan. I was in Iraq when all my old high school friends were having their senior prom.