According to what I saw in the newspapers this and last week, immigration are going to be looking for illegal workers here. Honestly, teachers aren't usually targetted as a rule, but are not immune from these kind of checks. We'll see.
Please be careful, if you are posting ads here, earning outside of your legal contracts, or even in a nearby City that's not your own. You never know. Perhaps just a telling off and a fine. Deportation can be a choice. The manager himself will tell you that this site is monitorred, and the odd ad placed to catch out the odd Waykook.
Check the Herald, Times, or Ilbo for confirmation. These checks will bring income for the authorties; fines for the Hakwons, parents; or Person working so, and are a great chance to show off for them, don't be a victim of their success, and your own.
Your face is likely going to all over newspapers and TV, it could be embarassing. Sometimes, no respect for privacy here, the laws can be overlooked for foreigners at times.