Standards of dress vary from school to school but in general, most schools expect a "respectful" casual appearance. It is always safe and good to wear semi-formal clothes.
Keep in mind that you are trying to dress in a manner which presents you as a serious teacher to your students, students' parents and business directors.
For male teachers: collared shirts, dress shirts and sweaters with a nice pair of slacks would be ideal. Some schools may ask you to wear a tie, however by no means are they required.
For female teachers: collared blouses, dress blouses and sweaters with a pair of slacks or a nice long (knee-length) skirt would be the best.
Avoid mini-skirts, ripped or torn jeans or t-shirts with offensive slogans, extreme haircuts, tattoos, extra piercings.
It may also be good idea to bring at least one business formal suit or outfit along to Korea, as you never know what occasions may pop up. The better prepared you are, the easier you’ll be able to dress appropriately for any situation.