I have been advised that a thru hike can cost between $2000 and $5000 depending on how much gear you have to buy, so earning some cash will be a serious challenge. You might consider a Rayway quilt kit instead of a sleeping bag - I don't have one yet though have a couple rave positive reviews from someone I trust. The quilt kit is a third what a bag costs (but you've got to have a sewing machine...)
I applaud your enthusiasm and won't throw cold water on it but please carefully consider that "Who cares though" when talking about missing the last 2+ months of your Senior year in HS. I'm doing the Cool Breeze flip-flop and will set out in late May '11 SOBO from the AT Train Station at Pawling. If you did something like that, you would still be doing a continuous thru hike, be on the trail at a good time of the year, and (importantly) not goob up your HS graduation.