You prepare the class curriculum with your Korean partner so that important lessons can be explained in Korean. Korean is not required to perform this task. Speaking English or speaking any other language requires the speaker to say things in a way that the listener can understand. It is also important that the listener tries to understand what the speaker is saying. If this can be accomplishing, you have already accomplished half of your job.
Appearance is an aspect of Korean culture that is very important.
The way that you dress and personal hygienic cleanliness represents the schools image and the perspective country you are from. Koreans are usually conservative in their dress, especially in work situations. Facial hair should be neat and well trimmed. However, you will be teaching children, thus a formal suit and tie or prom dress will not be needed. Thus, as long as you look like a professional teacher, you will be dressed and ready for work.