I'm doing 40 hours over two weeks, so I am not getting paid extra. This is within the regularly scheduled time they pay my salary, so it's all good IMO. Nothing unfair about it.
I have never signed a special contract for my before/after school classes, or the winter camp I am doing.
If the 27 hours is in one week, they are required to pay you 5 hours of overtime 5*20,000= 100,000 won extra. If it is not in one week, you shouldn't be paid any extra.
Hopefully you get to leave early after you finish teaching your camp. That's my hope!