>>3 Depending on where they live, going back to high school may not be an option.
>>1 OP, if you want to work a job and are in America, focus on getting your GED right now. Everyone I know who dropped out got one and hasn't had a problem finding work since. It's considered equivalent to a traditional diploma at most places.
As for making money on the side, understand that you're implying you'd like to run your own businesses with that artwork. It's a big undertaking that you don't go into without already having something to fall back on. Have you even studied your demographics? Done any test marketing? Planned any distribution channels? Just running into drawing and printing is going to be a dark money pit.
(Note also that it's illegal to make a profit from doujinshi without a proper license from the copyright holder. It's only put up with because it generally goes under the radar.)
Look for things that you can do with no overhead for now. If you're guaranteed stable internet, look into digital things. Are you a good artist? Sell prints (that do NOT have copyrighted characters, etc. on them) online. Even dA has print-on-demand features. Are you a good writer? Make an ebook out of your (NOT FANFICTION) stories to sell on Kindle, Nook, etc. Are you over eighteen and skilled at anything in particular? Consider doing freelance work for clients at sites like Upwork.
You're gonna have to do some research and explore the world. It'll probably be a while before your skills have developed to the point where you're going to get paid a living wage for anything, but you have to start somewhere.