The problem with the math curriculum is how much ground high school kids are expected to cover in one or two classes per year. Just look through older posts: algebra; polynomials; logarithms; stats; calculus; trig. That is a massive amount of material to cover. Further, since math builds upon prior concepts, if you had a teacher who skipped over part of the curriculum or you simply had trouble with earlier material, you're boned once you reach the more advanced concepts -- and things can get exponentially worse unless you either get tutoring or have a sudden epiphany.
I actually don't know what the solution is. I know a couple old school Ph.Ds in biology who have had to take crash courses in stats the last few years as they work through DNA analyses. Their joke is that they went into biology because it was considered math-light 25 years ago. But then, I also know people with solid math backgrounds who stumble on figuring out tips (it's not just the % -- there are social norms involved that influence the calculation). Most math curricula are light on doing everyday math mentally.
If you breezed through math in high school? That's freakin' awesome. I honestly wish I was better at higher math -- my job options would've been wider post-graduation.
But ... we're taking an accelerated math curriculum and throwing it at everyone, regardless of ability or, importantly, regardless of prior education. The one size fits all approach is kinda' crazy in a subject that, essentially, builds a scaffold from scratch.
Anywho ... with regard to "useless" classes like gym and the arts ... gym and music have pretty solid evidence showing they help raise academic scores (especially with regards to boys and doing something physical). Ditto for having green spaces for kids to spend time in during the day. Humanities classes, done well (trust, just like math and science, they often aren't), also teach critical thinking, but of a type that places value on being able to read emotions and placing events in context. The emotional IQ thing, as current thinking holds, is essential in making effective teams -- http://www.nytimes.com/2016/02...
Basically, we should be teaching kids 50 hours a week, giving them time to burn off energy, in environments with green spaces, with fully involved teachers, including individualized learning regimens (with private tutors, as needed), with music instruction (especially in groups) all in a cost-effective manner. IME, we're kinda' asking the impossible.