Korean university students seem to think that walking into class 30, 50 or even over an hour late (in a 1.5 hour class) is acceptable. And they want their little attendance mark when they do it, because if they have over a certain numbers of absences, then they automatically get a failing grade according to university policy.
I hate this. I loathe lateness. LOATHE. It's actually my #1 Pet Peeve, and the first year I taught at a uni in Korea, I felt like I had no control over this situation and so the students took advantage of it. They would wander in and out of class continually and disrupt any kind of vibe or activity that we had going on. It was extremely frustrating.
So, I made a new rule. You need to be sitting in your desk before the class officially starts in order to get your point for the day (a stamp=1% of the final grade). Then, I lock the door after 10 minutes to prevent any late-comers from even trying to come in. Because I'm so kind (!), a few more people who are 1-9 minutes late can come in as we're playing the daily warm-up game but they won't get their point for the day. And to minimize the annoyance even more, the first thing I do every day is play a 5-10 minute warm-up or review game so that a few late students don't interrupt everything or miss vital information that they'll need to further participate in the rest of the class. They just sit down, and relax until the game is over.
Do you hate something about teaching at a uni in Korea? There might be a solution to your problem that you can implement because you'll generally have full control over how your run your classes.