First of all, good for you! I think it's great that you want to go back to school. Whether or not you can attend classes and at what grade level depends on the rules of your local school district. If I were you I would start by simply placing a call to your local district or high school. Just call them up and ask how to proceed. I think you will find that people are happy to help someone who, like you, wants to help themselves. <br />
One of my sisters dropped out of school when she was in 8th grade. She went back to school when she was in her late 20's through her local school district's adult education completion program and ended up with her high school diploma. Whether you want to get your diploma or your GED, there's always a way to advance educationally if you are persistent and follow through. You can do it! My grandmother gets a degree every 25 years - she got her first college degree at 25 and her Ph.D at 75 years old! It's simply NEVER too late to learn and I think it's terrific that you are eager to do so.
Ask your local high school. Not sure what the law is these days, but when I was in high school the state law was that they had to take you up to the age of 21. If you remember any of your old teachers or guidance counselors, get back in touch with them. If you're sincere, I'm sure someone in the school will be happy to help you.
I don't think that a GED alone gets much respect in the job market. If you are highly intelligent person , you can take the GED and then move on over into some college classes. Even if all you do is something like community college on a Pell grant, or vocational school, well that would get some respect in the job market. If you are not intellectually gifted and your grade average is low, I think skipping four grades would not be good. Personally, I would homeschool, the average curriculum takes 3 hrs a day for a high schooler to complete. Then get a job , hopefully one that will give experience in the career path of your choice. That's what I did, my last 3 yrs. of high school were homeschool. I graduated 5 months before everyone else my age. I hated the social environment in high school. but I enjoyed the people I met at work.
Are you in the US? Since you're 18 now you might want to look into getting your GED. I think there might also be an option some places for an Adult High School Diploma (however I think it costs quite a bit more than the GED testing fees). I dropped out at the beginning of junior year and by the following year I wanted to go back to school. So I got into this program to get me back into my high school but I would have been 20 before I graduated at that point so I decided to get my GED instead. Good luck to you.
What stuffducks said. :P
Should be easy enough, there will be lots of different entry levels, depending on your abilities - just go into a college and talk to one of the tutors about it.