Is it possible to enroll high school at 20?

It's possible, but you will do a lot better to enroll in a community college. Take acting, civics, and personal finance management. Those are the things you need to know and no high school in the country teaches them. Get "Dress For Success" by John Molloy and consider all his advice for students, remembering that the book is about appropriate clothing, not expensive clothing.

When you read, instead of looking at one word at a time, look at two words. That forces you to stop talking to yourself, so you are no longer limited to reading as fast as you can talk. You probably talk about 100 words per minute (WPM). The average college graduate reads 300 WPM. Just by looking at two words at a time you can read 600 WPM. Then by looking at a whole line at a time you can read much faster. The big benefit is that your memory expands along with your reading speed so you can remember the large volume of information you are taking in. With a little practice it is possible to read an entire page in two seconds with near total recall of the information on it.