What do Korean high schools teach? Is it as difficult as China?

There are all kinds of high schools in Korea. You mean ordinary high school?

General high school courses are.

When I was in 1 grade: Mandarin, English, Mathematics, Sociology, Physics, Chemistry, Geography, Biology, History,

Music, art, sports and moral character are basically the same as those in China.

(It depends. Some schools also have religion or something. )

In the second grade, you will be divided into arts and sciences, which is basically the same as in China.

Grade three is basically the same as in China, preparing for the college entrance examination.