I think some resentful high school students that had a hard time often reflect American schooling poorly. I didn't have a supremely negative time (with the teaching at least) and I went to a standard 8:00-2:30 school in Massachusetts that had hour long blocks. I think we need to start taking more negative responses on the Internet as the extreme and not necessarily the norm.
Massachusetts seems to be consistently ranked as having one of the top public school systems in the US.[1][2][3] That might explain part of the discrepancy between your experience and my own.
(Edit: In fact, it seems MA would be pretty competitive globally against other country's schools - 9th in math, 4th in reading, 2nd in math. On the other hand, the worst U.S. states are on par with Bulgaria and Kazakhstan, among others.[4])
I'm not sure how to concisely respond to this as explaining everything right or wrong with U.S. public schools would be tall order. At the same time, it's likewise hard to believe your list would be the expected knowledge at graduation of a median high school student rather than a top-tier one.
For one thing, most items on the list wouldn't even be covered by U.S. standards (i.e. Common Core). Speak 3 to 4 languages fluently? The requirement to graduate in my state was 2 years of a foreign language (my school offered Spanish, German, French, and Latin, but options vary from place to place). Even assuming two years of a 55-minute a day class, 180 days a year, was enough to become fluent, that would leave you with two.
Similarly for science, you're required to take some basic science classes - intro chemistry and biology were the only required ones at my school. You wouldn't touch organic chemistry unless you took an AP (advanced placement, for college credit) class, and the teacher decided to cover it. I did take AP Chemistry and received college credit - we still didn't touch anything from organic the entire time, and I couldn't have even told you what a functional group was when I finished. Physics is just an elective and calculus-based wasn't even offered at my high school, so the majority of students aren't taking it.
Understanding special relativity? Quantum mechanics? General relativity? The highest math requirement is algebra 2, which might include a brief intro to how to add and multiply matrices. So exactly what level of "understanding" are we talking about this hypothetical median high school graduate having, in any country? General relativity is advanced enough that most physics majors won't even cover it beyond a superficial introductory level. You'd need a decent understanding of differential equations, linear algebra, and tensor analysis to tackle that kind of stuff, and the highest students could ever typically go (without the administration bending over backwards for a few particularly bright ones) is AP Calculus.
So I really have trouble believing the list of expected knowledge described in the GGP is anything other than a wish list for the best students. U.S. public schools may not be great but even I wouldn't criticize them for not meeting those standards for typical students.
That aside there are some real problems. Where I graduated HS, about 15 years ago, things were divided into the Honors program and Vocational program. The honors program was where you basically got what we might think of as a normal high school experience, teachers actually expected at least a minimal amount of work from you, and the better students had the option of AP classes. The vocational program was for everyone else. I knew people in it who didn't turn in any work their entire four years of high school. The classes were made to maintain a reasonable graduation rate despite having to deal with a large number of students who simply didn't care or for whom everyone had very low expectations. Those students might take two years of Spanish, but they weren't expected to be able to speak or understand it in even the most rudimentary way. Forget about actually learning algebra or knowing how to "solve for x." Even at the community college I attended, there were students who needed some science classes to do nursing who couldn't grasp the concept of atoms being made up of protons, neutrons, and electrons, and therefore couldn't get credit.
This is just how it is for a large portion of the populace - the really bad cases are far worse. My girlfriend is currently a student teacher at one of the two lowest performing schools in our state. She has immigrant students (in junior high) who not only don't speak English, but can barely read or write Spanish either. It seems like about 15-20% of her students would simply fail if she didn't take steps to try and get them to the minimum passing grade. But would it be better to hold them in the 7th grade indefinitely? Or to pass them along to a higher grade despite having only rudimentary understanding of the material outlined in the standards?
I know one thing she has changed her mind on recently, with actually being a teacher, is that it's possible for every student to get an A. It would take massive interventions beyond what a few teachers are capable of to turn around students from poor, rural, impoverished communities, who sometimes have histories of assault and abuse, and who have been perpetually falling behind the ideal outcome a little more every year. For the really poor outcomes, or just the suboptimal ones, it's a lot more complicated than just "what do you do in high school all day long?" The cases of having students primed for learning but not given enough resources are far fewer than the ones where schools are taking disadvantaged people who have already fallen behind and trying to make the best they can of bad situations.
I'm no fan of the U.S. public school system, but even I can't blame it for not being able to get students up to those kinds of standards before graduating.
What do you do in high school all day long then?