First of all, why do you want to go back to school? It defies the purpose of quitting! I'm not saying that you're wrong for quitting, nor am I suggesting that you shouldn't learn any more than your current knowledge capacity holds. Though, you really don't need schooling to learn.
As for GEDs, they're not so hot at all. Sure, they're alright, but you're better off going with another program--getting a CDG (or something like that, I can't remember what it's called off-hand). You take some work home from a school, do it, bring it back, get tested there, occasionally, and should you pass everything, you get an actual high school diploma from whatever school you do this testing in (certain schools don't hold this program). My mother has done this and has a diplome in the West Islip High School and she wasn't even in that district.
And for my curiousity, how did you get out of school so early?