You're right. Korea has zero tolerance for pot and foreign smokers. Better to fess up now if you have any previous registered conviction (misdemeanor or bigger), you don't want to be waiting everyday in korea for someone to find out. Most schools directors (my opinion) act when they see something wrong with the behavior of their English teachers - first thing they do is to check if their diplomas are real and probably now they also double check if the teacher had any convictions, pending cases from they come front. Consider the visa form you are filling now as an affidevit, if you lie - you are lying to korea and if they find that you lie, they can jail you and deport you asap.