Stay in school and stick it out, unless as chaosx said you plan to enroll in college and at least earn an AA degree.
The reason is that no matter how intelligent you are or how solid your business plans may be the world of business in general views it as dropout=quiter/loser. You will find it harder to get any type of business loans and generally have it tougher.
I speak from experience.
It may feel like a solid plan now, but you are still only 15. I don't mean that in any type of negative way at all, I was 15 once too.
Just that at your age time seems to just ccrraawwll soooo sloowwwly, but in reality ten years will go by before you know it. A lot can and will change in that ten years, and you will still have a long and full life ahead of you. During that time you can earn a high school diploma and a college degree in horticulture with a minor in business and then really kick some ass with your business plans, and still start them now and grow your business part time until you have the credentials to be taken seriously in the world of business.
Trust me, it makes a big difference.
Making a business a success is very difficult, especially in todays economy. Graduate and have a backup plan as you pursue your dreams.
At the very least you can look back and not feel as though you ever quite anything and gain a real sense of pride and accomplishment.
In short, earning your high school diploma says a lot more about the type of person you are and sticking with something a lot more than it does about reading, writing and arithmetic.
__________________

"the terrible thing about hell
is that when you're there you can't even tell
as you move through this life you love so
you could be there and not even know
but you say so what I'm doing just fine
the irony is that it's all in your mind
and that's why hell is so vicious and cruel
but you'll just go on an oblivious fool"