Is it possible and realistic to learn math at high levels outside of school? And if so, what resources should I use? As a high school student, I really don't think the curriculum is fast enough for me.
Of course, I've done that all the time. The key point is to enjoy what you're doing. Then it will all become easy.
One of my most interesting studies was with the book "Calculus: Early Transcendentals" from James Stewart. Pretty interesting.
For calculus and linear algebra, Khan Academy with 3Blue1Brown would be good as as an introduction.
If you like, you could go into Competition math, and see if you can get into the Olympic stream. It’s always good fun and your learning of math significantly improves.