First and foremost, there is the language barrier. Obvious, no? You are not required to know ANY Korean when taking a job to teach as a native English teacher. You are here to speak English anyway right? While this is certainly true, you need to realize there will be a lot of confusion between you and your students. You have no idea what they are saying and they are baffled by the sounds that come out of your mouth. You may both pick out a word here or there, but there will be a considerable gap in understanding. You can't even express to them that you don't understand Korean, so they continue to try and communicate you with slowly spoken Korean that you have no foundation to understand.
Be predictable, be patient, be fair, and do not take things personally. Kids may not participate, ignore you, or even mock you, but you need to treat each day as a fresh start. This does not mean let them get away with bad behavior, but rather after dishing out the prescribed repercussions don't hold a grudge. Kids have lives of their own and you may be the only person they can take it out on. Or maybe they hate English because they are tired of going to hagwons (private tutoring classes) after school instead of relaxing or playing. You need to at least provide them with the chance to improve, and it's their choice to take it or not.
Think of it in terms of a child and their pet dog. The child will talk to the dog and get frustrated because they don't see why the dog can't understand. However the dog is always happy to see the child and willing to engage when the child is ready. Try to emulate the dog's attitude. Even if a student had a bad day yesterday, at least they know you'll happily include them in today's discussion. Be forgiving.
I personally encourage the smart asses in class. If they have a silly answer for a project, I'm totally for it as long as they follow through in English. If you want to draw an ant asking "May I eat this human?", that is a great answer. You want to make a zombie pig out of clay? Go for it! It may not be the exact assignment but the kids really are surprised when you tell them that you like their idea and they can do it. They often really enjoy the assignment afterwards and put a lot more energy into their work.
It mostly comes down to kids being kids. You may not be able to communicate well, but they still want your attention. Sometimes it's by misbehaving, other times running up to you after school to say hello. You will have your favorites, and not your favorites, but they don't need to know that. In order to really learn they need to enjoy some aspect of English, and the teacher's attitude is good place to start.