For this age group, there are some things to be borne in mind: Most of these students, do not stop learning outside school but would still attend extra-curricular learning sessions in the afternoon. It means they would have a very tight schedule, and that sometimes can be very demotivating.

I would therefore suggest a number of activities that might perhaps make a difference to their ordinary school life: Use music and pop-songs as much as possible to complement your lessons, also show movies (videos) that may be of interest to this age group, and take them out for a walk and ask them to show you the city and explain places of interest to you in English.

You could also ask them to teach you some Korean language.

You can't do this too often but it may be a nice break for them at certain times......

Good luck!