Congratulations for having gotten this job. You are lucky that the job came up, but no doubt capable and qualified. I want to second what the other commentator said. Yes, those high school kids may be at a higher level, more focused and more demanding, and if they are in the gifted category, that will be more of a challenge but perhaps a lot of fun too.
This year, I've been teaching Dutch teenagers, in the U.K. They are somewhat deceptive, in that they can be very glib and fluent, and seem to know everything. But I asked them to define certain words that they seemed to know, and well, they didn't know. In order to teach them, I had to adapt the material by adding more challenging debate questions. In classes where there was a bit of reading and a debate, I would extend the debate, adding in different scenarios, and asking students to give a critique of what their neighbours had said.
Depending on the class, you may be able to add authentic materials such as articles from the Guardian newspaper about Brexit, or even something about business English. A business English teacher told me that the intermediate to advanced level kids I was teaching would probably also love learning business English. As teenagers, it is a new thing, and it will make them feel more grownup.
The high school in Busan is likely to have kids from intermediate to advanced level. If there is more than one level in a class, you can adapt material by adding extra information and questions for the advanced kids, or giving extra clues for the slower kids.
High school can be more challenging in Korea, perhaps. My experience at a Korean high school was challenging. It involved teaching at 3 technical high schools, of largely non-academic students. It was tough, because there were very wide differences among classes, and some students were very troubled and bitter. I was accused of being too like a professor even though many classes were fun and lively.
