The Oxford English Picture Dictionary, either monolingual or English/Korean version plus the teacher's book and workbooks.
I'd probably go for the teacher's pack from GenkiEnglish and a few sets of early readers, leaning towards the lowest level available -- if you think you can get away with assigning reading homework. The kids generally won't read the books unless they're encouraged to do so. Also, dictionaries -- if you do some reading/writing work in class and think they will use them.