Quitting your education to write is never a good idea. You say yourself that you're an "okay" writer. You won't improve if you don't continue with your English classes. Honestly, every class will be important because the best ways to improve is to write...essays, reports, anything that keeps you writing.
And, as cruel as it sounds, you can't get your hopes up with your dreams of writing professionally. Very few people are successful at this. I read somewhere that there are over a million hopeful writers out there sending a manuscript in every day. Of those, maybe only five are accepted. That's not very good odds...especially if you don't at least have high school level skills if you plan to write teen or adult novels and screen plays.
Do you know how to structure a novel manuscript? What about a screenplay manuscript, which will be totally different from that of a novel? Just going in and slapping a bunch of words down won't cut it. You need to understand that you will have to have these skills because an editor will throw your hard work right into the trash if they aren't set up exactly as they need to be. You have to understand that, in screen writing, you have to go to the minutest detail...including small objects sitting on a table or the style and color of buttons on someone's jacket. It's very tedious and time consuming.
That's why many writers choose to finish their education. Many of us move into college and take writing courses and workshops to hone our skills. Of course, they aren't absolutely necessary, but they can often be the deciding factor in whether or not you can actually produce something worth selling.
And here is another fact about writing. It's NOT easy! People think that writing is just slapping a bunch of words on a page, and they think "Geez, I could write that!" But the fact is, it's not easy, and there are a lot fewer people who are actually great writers. Most are what I call "gobble-di-****". It's not worth the effort to run your eyes across the page. Writing is not just a talent. It's not worth anything unless you are willing to take the time to sit down and write everyday. Those people who say you don't have to may be right to an extent, but a real writer doesn't put their work away for weeks or months at a time and then bring it out and expect a great novel to finish itself. When they're not writing, they're proofreading and editing. They may even be exploring the world around them with works from other writers or with observation and critical thinking. You can't be lazy. This is the mistake most people who think they are writers, or dream of writing, make. They think that writing is all about churning out a great story, and not having to survive in the meantime. The truth is, even if you are published, it could take years to become a success. With no education, how will you survive in the meantime? I can guarantee, if you aren't making an effort to do something fruitful with your life, your folks aren't going to tolerate your sponging off them forever. And your friends and family won't help out if you can't make the rent!
Do you really want to be 45, looking back on your life, struggling at a "go nowhere" job while waiting for your big break? I can guarantee, you'll be lucky, in this economy, to be flipping burgers or slopping hash if you don't have at least a high school diploma. Heck, college grads are being forced to work as underpaid slaves right now! What will a "I dropped out of high school because I plan to write" get you?
You need a back up plan. You need a job to support you while you're receiving rejections, until your work sells (if it ever does). How do you expect to get that, and how to you expect to publish a novel or have a screenplay picked up, if you don't even have the perserverance to finish high school? Look up the bios of some of your favorite authors or screenwriters. I can guarantee you that not one of them doesn't have a story of what they did to support themselves while trying to be published. Finishing school takes persistence. Writing takes more. If you can't find the willpower to finish school, you'll use that first moment of writer's block or the first rejection or the first obstacle you hit as a reason to quit writing too.