I remember you posting about this speaking test a while back. It's a nice take on how to do a speaking test and sounds like you put a lot of thought and effort into it.I guess the students decided it's too hard and can't be bothered. Maybe it's the first time they've encountered this style of speaking test. It's a shame your CTs haven't backed you up a little bit more. But yea, don't worry because you've done your job, you can't do much more.My speaking test is five simple questions and I give them two or three example answers for each, about a month before so they can prepare. No one does though. It's one of those times when you have to accept that no one really cares and you just have to switch to zombie mode and sleepwalk through it and not worry too much.
This isn't a Korean thing in the least. Where I am in the States, give kids several days to build something, tell them which materials they have to bring, and find that they start on the day before it's due and complain that you didn't give them the things you told them to bring themselves.
Also keep in mind that your Grade 3 high school students are also studying and preparing for their 수능 all year long, and that's a lot more relevant of a worry than an English speaking test. As others have suggested, it sounds like you worked hard and prepared something, but you should also try to avoid getting frustrated that the students aren't falling in perfectly with what you planned. They have a lot of other classes, subjects, and tests.Consider shortening your time frame, like a few others suggested.