1. Because of Korea's Confucian influenced society, older people automatically get more respect than younger people. Sure, we respect our elders back home, but it's at a whole new level in Korea. In Korea, when you meet someone for the first time, one of the first things people try to find out is your age. This is to find out if they are older, the same age, or younger than you, because it can influence the way they speak to you. With people older than you (unless you're very close friends), you use jeon dat mal (using the 요 yo) with older people. Traditionally, at restaurants, the oldest person pays. While this isn't really the case anymore with young friends that are similar in age, I've experienced this with friends I have that are about 9 years older than me. One guy friend I have, who is around 34, and I'm 25, will pay for me and all my friends when we go out to eat with him. While the younger people will set the table for everyone.
2. Taking shoes off when you enter houses (and some restaurants). This is probably not a surprise to most people, but when I first went to restaurants with on the floor seating, I was surprised I had to take my shoes off. If you're wearing big boots sometimes it can be a pain to get them on and off really fast. So that might be something to keep in mind if you're going out to eat with your school, because typically they like to go to more traditional, nice places where you sit on the ground.
3. Chopstick etiquette. This is probably common sense, but it is very rude in Chinese, Korean, and Japanese culture to stick your chopsticks vertically into your rice. Instead when you're not using them just put them on your napkin or the table neatly (not crossed), or rest the "mouth" part of the chopsticks onto your plate. . Additionally, it is okay to eat out of communal dishes in Korea with your chopsticks. In fact, family-style dining is quite common. It might freak you out at first to see people sharing the same food with the chopsticks you put in your mouth, but you get used to it fast. If you don't want to do that, you can serve yourself first and put the food right on your own bowl or plate. But sometimes, the plates are very small or there's no plates at all, so you have to just share the food communally.
4. Drinking. Koreans are big drinkers. A lot of Korean people will drink alcohol with every meal. This being either soju (a liquor similar to vodka), makgeolli (rice wine), or maekju (beer). There are also some drinks that are standard with certain meals. Such as maekju and chicken (hence, chimaek) or makgeolli and pajeon. It is not polite to pour your own drink, and the youngest should poor the drinks for everyone else. If you need more to drink, just indicate that you would like some more to your neighbor. Finally, if a toast is made, turn your head to the side, away from your elders, while you drink. If you're not a big drinker, it's okay that you don't drink, just tell them your reasons why and do the toast with water or tea instead.
5. Excuse me! In Korea, it's completely normal in restaurants to yell "저기요" (jeo gi yo, excuse me!) to get the server's attention when you need something. If you don't yell this (unless you make very direct eye contact and put your hand in the air or something), the servers will not come and check on you. The servers will only come to you when you say/yell jeo gi yo or if you happen to be at a restaurant with call buttons on the tables.
6. "Service" : A completely wonderful perk about restaurants in Korea is that often you get "service". This means some free food usually, but could also be an item. A lot of times the "service" I get is a free bottle of coke, or some free french fries or another side item. If you go to a restaurant regularly, or maybe you go and bring your friends back the next time, you're likely to get "service". It's great. I once heard a story from a friend who, before knowing about "service", argued with his server about an item he had received, because he had not asked for it and didn't want to pay for it. Little did he know it was just "service', and with the language barrier, the server had a hard time explaining it to him. So if you have an item put on your table and the server says "service", you know what it means!
7. Bowing: In Korea, you bow instead of shaking hands. Your students will bow to you every morning, and you should bow to your fellow coworkers at your school and your boss/principal. Additionally, Korean people bow to their friends when they see them for the first time that day or when they leave to go home. Korean people rarely shake hands. I was expecting this when I moved to Korea, but still, when I see my students bow to me first thing in the morning, most of the time I bow back without thinking. Then I remember that I don't have to because I'm their teacher and a lot older than them. But it still feels weird to me to get bowed to and not bow back.
8. Red writing: Don't write people's names in red! It means their dead! Korean people normally write people's names in red when they die on the funeral banner or the family register after a relative passes away.
9. Personal space. In Seoul especially, there are so many people so there are very different perceptions of "personal space", especially on transportation like the subway or bus. People will push and shove you in order to get around you, or to make sure they get onto the subway. The worst perpetrators of this are the elderly, who will blatantly touch or push you with their hands in order to get around you. Sometimes in these instances the elderly people can seem very rude, but you have to understand that its part of their Confucian culture and they expect to be respected no matter what, and they are also accustomed to a lack of personal space. A lot of younger Korean people I have talked to have said that the younger generation is a lot different from the older generation, so this may not always be the case.
10. Sharing. If you bring a snack (such as cookies or chips), it's polite to offer your snack to your coteachers. In return, your coworkers will always share with you. I find this particularly true with my older coteacher. Almost every day, she offers me a little bit of some snack that she brings. I usually feel bad because I don't often bring snacks to school. But every now and then i'll try to bring something good back so I can share with her. Korean people love to share. Sometimes even when I go hiking, after I reach the peak and sit down with my friends to relax and eat lunch, Korean hikers nearby will offer us their magkeolli and some of their lunch (Korean people usually bring a whole meal when hiking, complete with magkeolli!) We found it surprising and extremely kind that complete strangers would offer to share their meal with us.
Finally, here's a funny link about things foreigners experience in South Korea http://oogeewoogee.com/part-2-15-funny-things-foreigners-experience-in-south-korea/_