Appearances can be deceiving. Secondary education is compulsory in the US, whereas it's not in every OECD country. In some countries such as Japan, high school is not only optional but you have to take exams just to get accepted into a particular high school. This effectively stratifies students such that you end up with much more homogenous classrooms and allows for more consistent teaching.

Many countries also offer vocational alternatives over academic classes at the secondary education level. While some districts in the US offer vocational "tracks", they still have to abide by the federal and state academic mandates, so they're not usually purely vocational.

Not that the structure of the US system isn't a problem in itself, but the fact that we have compulsory, academic-based secondary education that requires public schools to accept all students zoned for it leaves the US in a tight spot. We'll already have a lower graduation rate simply from people dropping out that would otherwise never have started if it wasn't compulsory here. But it's even worse since we don't optimize it to the students (such as the purely vocational options and competitive high school entrance acceptance that allows for a more homogenous student body and more tailored teaching that I mentioned previously)