I want to learn mathematics to extend myself.
I highly suggest that you begin with learning elementary number theory.
You will understand different proof techniques
Number theory is kinda like a puzzle, It will enhance your problem solving skills and will you will also learn a lot of different notations that is used in all over mathematics.
Unlike complex analysis or calculus, I find number theory to be very exciting and encouraging for students who want to learn maths at a young age like you. That's why you will find a section in the mathematical Olympiad challenge about number theory.
There is not a lot of videos that I found interesting on number theory on the internet.
However, I highly suggest to read this friendly text book
Introducation to mathematical thinking
It is very easy to read and it's one of very few maths books that you won't actually get bored easily reading it
Thanks for your interest! It's always nice to see such dedicated young individuals. Since you live in the US, I feel obligated to say that as a former student, the US's education system is pretty screwed up; learning here is based on age rather than intelligence. What you want to do is perfectly normal, as the math system is going too slowly for you, but I suggest that you or your parents keep negotiating and pushing the school to accelerate you in your mathematics courses. Even just one year ahead can be a great benefit. It took us forever to allow me to be able to get the school to recognize the geometry I was learning on my own, so I definitely know the feeling you have.
As far as resources go, you should try borrowing an algebra II/trig textbook from the high school to learn the basic materials of trigonometry and algebra (conics, logarithms) that anyone should know. From here, you could consider learning calculus, but it may be better for you and the school to wait until you actually have a class in the course. Instead, expand your mind in the other numerous and accessible fields of math at this point in your life. You should look at more number theory, (more) geometry, combinatorics, and even linear algebra, as these items are not traditionally taught in a US school (save basic geometry). If you're looking for books on these subjects, I recommend Dover Publishing (and any introduction/beginner books they have) as they are relatively cheap and of good quality.
Additionally, you could look at recreational mathematics, which are the maths of novelties and puzzles. These are meant purely to enjoy and have fun with. Again, if you are looking for books, look at Matt Parker's Things to Make and Do in the Fourth Dimension or any logic books by Smullyan. There are of course, countless others.
The worst thing you could do at this point though is stop. Keep pestering your school to move ahead and keep looking around the internet for things to learn. Best of luck.
Textbooks
The clearest way to learn maths is to get your hands on a good textbook. I find that reading a textbook is the best way to study any subject.
When I was in high school (grade 8 to 12 in South Africa), I started reading through a calculus textbook which had been lying around in my house. A calculus textbook will often summarize the key concepts of trigonometry, logarithms, functions and polynomials in the first few chapters, and goes on to discuss topics that would be taught in your later high school years and first few years at university such as limits, differentiation and integration. A calculus textbook will extend you however, and you may want to start off with a textbook written for the higher grades of school.
Another option, which is more geared to the pure side of the science, is to read a book about logic and set theory. That will accustom you to the notation used in proofs and in defining mathematical structures.
Online tuition
If you have a specific question, searching online can often give you useful papers to read through. Also, there are websites geared for tuition such as https://www.khanacademy.org/ .
EDIT: I have just clicked on an advert for the MIT courseware: http://ocw.mit.edu/courses/#mathematics and this looks very promising!