Why is the American education system so awful?

I had mastered all the math my high school had to offer by 10th grade. I spent the next 2 years doing nothing but basic review. I was not allowed to take college courses and such.

My ability to self-study was limited by the shear amount of bullshit homework, papers, and such that didn't actually teach me anything and were quite trivial, yet so numerous they'd take up all my free time in between school and the extracurricular activities required to get into a half-decent institution.

It's not. It's built in discrimination so that only the best succeed and contributes the social class order. It's a necessity in capitalist democracy.

Because everyone constantly judges each other I would imagine that discrimination would be necessary in any kind of political system.
Although I don't really know your definition of discrimination.