It's rather quixotic that your screen name is Michael Ehard and you sign your post Lucy James. Your gender would condition the response to your questions. Like it or not, young people value athleticism more in boys than in girls. No one can say whether or not you will be liked, that depends on you and how you present yourself to others. There will be a certain number of people who will think it's "cool" that you speak with an accent, but that sort of entre into a social group only lasts as long as it takes people to learn your character. But it may help you.
EDIT: Oh, that was cute. You changed your signature while was composing this post. I now suspect that either this is a bot, or you are not being honest. I have now lost all interest in helping you.
In the United States, the education system is not geared to your age, except vaguely. Whether or not you are 18 doesn't matter, what matters is whether or not you have completed courses which are equivalent to the requirements for graduating high school. I completed my requirements by age 16, but was not able to enter university until age 17, simply because of the papterwork i had to complete for enrolment, and to secure a loan for school. However, most schools in the United States require (or did when i went to school) that you complete civics courses (or whaterver they call them now). Those courses focus on an understanding of government and governance, so you would be expected to know or to learn a certain amount of United States history, as well as understanding how the government is structured and the relationships of the branches of government to one another. My cousin who lived in Chicago also had to complete an equivalent course for the state of Illinois. If you have met all other educational requirements for graduation from high school, then the civics course, or government course, or whatever they are now called, would be all that you have to complete.
This is a document from the Illinois State Board of Education
which explains graduation requirements. CAVEAT: that links takes you to a PDF document. If your internet service is through dial-up, you may experience difficulty opening the document. If you have broadband service, and your computer has generous RAM and a fast processor, then open the file and read what the state board of education has to say.