When working in schools it is expected that you will have a smart appearance.
Although there is lots of flexiblity in terms of dress code, basically your working place is a professional environment, therefore business casual is appropriate for class hours. For special occasions such as school open day or teacher's day, you will need to wear something a litte dressier.
Dress code for your orientation ( 8-10days) : The same principal above is applied to you. Teachers are free to dress casual when they are not in classes and on the field trip, however, business casual is suitable for classes or the opening and closing ceremony. QTKorea sends out more details by email before you go out to Korea