High school drop-out here.
Heed my very real advice as someone who has been there: get your GED. DO NOT GO BACK. You will waste another year of your life and you'll be nowhere. You'll also have to tolerate the high school mentality - at 20 years old you should be beyond that.
If you are intelligent enough to get your diploma then you are intelligent enough to get your GED. Sign up for it and get it THIS month. It's not difficult and just as valued as a diploma if you play your cards right.
This is how you play your cards right:
(1) Get your GED asap and apply to your local junior or technical college for fall 2009. They will let anyone in with a check book, heart beat, and GED.
(2) This is key: Apply yourself. Apply yourself at school and commit to getting a 2 year degree. Start every course at the lowest level. Trust me. Math, English, everything. Really work on learning the material and really concentrate on understanding the math, grammar, etc.
(3) Once you have a JUCO degree and a 3.0 (it shouldn't be too hard if you actually study - you'll probably have more like a 3.5) you have your meal ticket. You can get into almost any college. It may take 3 years to graduate from undergrad with credit transfers. I graduated in a total of 5 years.
(4) Do well in your undergrad work at the 4 year school. You'll likely start with a fresh GPA at 0.00 and you'll be in control of your destiny at this point.
I dropped out of high school in 9th grade. I began JUCO at 22 and finished JUCO at 24. I graduated undergrad at 27. I felt much more focused than my peers most of the time, as I had seen what failure can do.
I'm completing my J.D. from Florida this spring. You can do it! Good luck. Just get that GED 1st and don't worry about going back.