Maxwell Zhu, 18-year old fresh out of the high school experience
As a Chinese-American 18-year old who has experienced both education systems first-hand, I feel uniquely qualified to answer this question.
Based on my experiences, there are several major differences between American high schools and Chinese high schools.
Technically speaking, Chinese high school curricula are much more demanding and advanced. Most Chinese students must learn calculus, biology, chemistry, physics, English, Chinese literature and language, and most of it, especially the STEM classes, are comparable to college undergraduate level courses. However, much effort is placed on rote memorization, and all of this is tested on the GaoKao (高考), the nationwide high school exit examination that all high school seniors take over the course of 2 to 3 days in June.
American high schools, on the other hand, are less rigorous academically. Obviously, classes must still be taken in all of the core areas, but the material is not as challenging, nor is the culture as demanding of academic excellence. Furthermore, there is a greater emphasis, though in my opinion still not enough, on critical thinking, problem solving, and creativity. As a result, quality of instruction is more important in the US, where teaching philosophy, method, and behavior is extremely important, compared to in China where teachers are mostly responsible for getting factual information across.
Contrary to popular stereotypes, Asian students do not spend all day studying. They, too, have sports teams and clubs and social lives. However, in contrast with American high schools, Chinese high schools are not super big on those things. Whereas in the US football games are a big thing and there are athletic superstars in high schools, that's much less common in China because there simply isn't as great of an emphasis on athletics. Clubs are few in number and limited in nature. American traditions such as Homecoming and Prom are not a thing.
Chinese high school students do lead wide and varied social lives, though, like any other high school student. They'll go out with their friends to karaoke and dinner and hanging out downtown, complain about teachers and grades, and probably a few of them get drunk (no drinking age in China). They pay attention to fashion, sports, and entertainment. But because of the nature of Chinese high schools, more of this is discussed and expressed outside of school compared to what happens in he US.
Chinese college applications are very straightforward. Based on the results of their GaoKao (see Academics section), they either are accepted or rejected by the university they applied to. Admissions is completely and only dependent on this exam score.
On the other hand, college admissions for US high school students is significantly more complex. The admission process involves multiple standardized examinations (SATs, APs etc), grades, recommendation letters, extracurriculars, personal statements and supplemental essays, interviews, and lots of luck.
At the end of the day, high schools in China are a government-administered system designed to facilitate the education of the greatest number of people as possible. There is a large emphasis on unity and uniformity; all students wear a school uniform, each of them wears a red scarf associated with the People’s Republic of China (like we have our Pledge of Allegiance), all students take the same courses and the same tests. Grades are openly broadcast to encourage effort and academic improvement.
High schools in the US are an open system that provides a transition from middle school (where timeouts are still a thing) to college/university (where one makes life decisions). There is a greater emphasis on individuality and freedom; no school uniforms in public schools, students can choose which courses and their difficulty, to an extent, and there is a high degree of student privacy in the context of grades. Schools are arenas for socializing and, whether students realize it or not, learning how to communicate and interact with others in the real world.
They look different too! Chinese high schools look like government institutions. Note the large, gated entrance though which everyone enters and exits, the security booth next to it, and lack of indoor parking since students don't drive.
Guangzhou No. 47 High School - Wikipedia
US high schools, on the other hand, have multiple entrances, the gates don't look as foreboding (at least none that I've seen), usually feature a parking lot, and are generally more spread out than Chinese high schools.