The public school curriculum today was built around a need for literate men who could do arithmetic working in factories from the 1920s and on. That is why the core "skills" of public education are mastery of basic reading and math.
On one hand, reading and math are pretty fundamental to most of the world today. On the other, the original designs of curriculum were to repeat those two concepts for 12 years until everyone who graduated was certainly capable of them to insure any adult was a capable factory drone. Throw in some biased national history in social studies to create some cult patriotism and basic science so they don't blow up half the workplace floor by mixing the wrong chemicals and so they have a basic idea of how things move and that was it.
The average high schooler keeps graduating more and more capable of memorizing words and regurgitating them on fill in the blank tests. They are getting better, on average, at more advanced mathematics like algebra. It turns out generations of kids capable of doing math homework didn't dramatically improve the innovative potential of east Asian countries that focused hugely on that from the 80s and on. Japans school system is one of the most rigorous in the world, you have to pass entrance exams just for high school and getting into any university is itself often a several-year long job. But Japans economy is not seeing huge economic growth for all that laser focus on classical lecture and memorized testing.
And even then the current k-12 system does not serve those who aren't as quick to pickup on these memorization concepts and fundamentals for reading and math. Huge numbers of kids are "forgotten" in the public school system and are pushed through despite being borderline illiterate. Because grades are age based and not ability based, if you are either too fast or too slow at memorizing the material instructed at you public school will be an awful experience.