If ever you want guaranteed laughs from a Korean (and you’re a foreigner), speak in their language.
During class, I was explaining sentences with the same meaning as “I don’t know”. After listing 3 or 4, I go “It’s like when you say mola in Korean.” At first the kids are a bit startled and give you a confused look, then if you repeat it, they’ll understand what you’re saying and laugh. This one little girl, who I’ve never really noticed before, she’s always in her corner, all quiet, laughed super hard both times I said it. Apparently her Canadian teacher saying “I don’t know” in Korean was the funniest thing she heard all day.
After a school day, I was walking out of school, my hands still wet from having washed them and I crossed some kids who wanted to shake my hand. I tried to warn them about my wet hands, but it didn’t really work out, so after shaking and surprising the kid, I pointed at my hand and said mool (water). They started laughing and giving me a thumbs up.
This doesn’t only apply to middle school kids. I met with a Korean English teacher Saturday and after our meeting I asked if I could say Suwon yeok imnida (more or less “is this Suwon Station?”, I think) to ask if this bus goes to Suwon station. I got the same shock-then-laughter reaction from an adult. Once she stopped laughing, she told me I could probably speak Korean and that she only laughed because she didn’t expect it at all.
So there you have it. Learn a few Korean words, throw them in every now and then during class, and it’ll give everyone something to laugh about. Plus you can work in that the students have no need to be ashamed of speaking English since your Korean is more or less one million times worse than their English.
EDIT: Well, turns out my Korean isn’t as “great” as I thought it was. I should have said Suwon yeok imnika? What I said translates to “this is Suwon Station”. It also turns out that when she told me I could speak Korean, I was getting a taste of Korean sarcasm. Their sarcasm is apparently like their liquor; it sneaks up on you and leaves you devastated. And this, my friends, is why you shouldn’t listen to someone who can’t speak the language for language advice. I will mislead you.