I also have to teach writing for a semester at my high school but we have a set textbook from which to work.  However, my fellow NETs and I discovered at the end of last semester that even following the textbook, we were basically repeating what the KETs were doing in their lessons which explained why our students always looked so bored. 

If you have to teach academic writing, it's difficult to make the lessons interesting - conversation classes are easy to make fun and interesting lessons.  However, if the school is just wanting you to teach writing in general, it provides a lot more freedom for creativity.  You can do lessons on newspaper writing, emails, text language, postcards, letters, feature articles, etc and it's pretty easy to make fun lessons for these genres.  Also, it depends on the level of your students.  I teach at a foreign language high school so the proficiency level is quite high which means more time teaching structure and editing than the basics. 

Of course, teaching writing also means that you're going to have a LOT of marking.  I teach 16 hours per week and mark for an average of 30 hours per week. 

This is going to be tricky. There isn't a large archive for our sort of work on here or anyplace I can think of. They want you to teach writing? Really? I have heard a few cases of this, but I have not seen many. I studied Literature and writing at University and almost NONE of that helped me coming here. What level(s) of English are they? And are you teaching the entire school? I am teaching the entire school, so I only see 300 (1st years) students each week and the other 600 change every other week. I see some of the 1st years 2-3 times a week due to after school classes.

Let me know their ages, sex (boys, girls, mixed, academic/vocational, etc. I might be able to toss you some ideas and whatnot.

They are highschool, but I don't think their English is very advanced. I teach 15 classes a week, 8 boys classes and 7 girls classes.

They get alot of common mistakes, like directly translating Korean to English, so sentences are kind of structured backwards. Alot of them have trouble in writing in the past/present/future; a majority of them can only write in the present tense.

I have seen alot of highschool level lesson plans on the website, and I think alot of them would be too advanced for my students. So I think they would be like high-middle school low-highschool calibur...but my teaching experience is less than one month, so I can't say I am entirely certain either.

I just remembered another good writing game. This can be played for an entire period and with a large number: Sentence Auction. I found the game on Dave's. Here's the link: http://www.eslcafe.com/idea/index.cgi?display:913441548-3364.txt

This is great for testing them on tenses and articles.

What a challenge!

I'd start with journal writing topics for the first ten minutes of each class. Collect them and read them for the first week or so. This will give you a good idea of what to focus on.

Basic topics could be stuff like what makes a good boyfriend or not including your parents who is your hero and why.

sasez's journal topics are a good idea. In case you didn't know, when people say you should have your students keep a journal, they don't mean a diary of their day to day lives. For most high school students you'd get the same answer every day. The journal topics is a good way to get them thinking in English and putting their thoughts down on certain subjects (far more interesting that get up, shower, go to school...etc).

A good way to start off in class is with jumbled sentences. It gets them used to putting sentences in the correct order. I like playing a betting game with that in which the students are given fake money and have to gamble on whether sentences are correct or incorrect and actually have to correct them if they need correcting. They usually get quite into it. Work on sentences first and then over the course of the year, get them writing paragraphs and then, who knows? It takes a lot of time though.

If you have friends in different parts of the world, you could, with their permission, have your students write postcards to them and have your friends reply to a few of them. The kids would go mental for that. There's tons of stuff you could do. Forget about showing videos and get on with some actual teaching. Technology seems to be blinkering teachers these days.

Yeah, find a teacher in another country and organise a pen-friend swap. Writing is one of the most tedious things about English for most Korean students and most hardly even know how to write formal prose in Korean. However, find pen-friends in other countries and suddenly something that couldn't be more boring couldn't be more exciting. Last year I got my CT to hand back a bunch of replies I received from an American teacher I met on-line. In my CT's words it was 'the best idea ever' and she said she's never seen students so excited to get their hands on something that's in English in 25 years. The American teacher said her students were just as thrilled.

Finding this two years later has been a joy. I'm in the same situation as the OP. Thanks for all of the great advice!

I have had to teach Academic writing, and yeah it's hard to make it interesting.

But if you're lucky and you don't have to teach Academic and just focus on writing skills- you can look into more interesting activities (instruction writing, recipe writing (both real food and metaphorical personality recipes), letter writing, postcard writing etc)

I often find a trick is to take the conversation lesson and try to use the ideas/activities and just turn the focus to writing. I.e. the Describe and draw: For my conversation class we will do it in real time, with a partner describing as the other draws. For my writing class, they write a paragraph and then swap. Or for instruction lesson: conversation will have them again do it in pairs or groups in real time as someone follows the instructions (to get through a maze or to build soemthing) but writing class will have students write it down then swap and read and follow another students' instructions

If you take your topic, and google for "[topic] lesson plan" or "lesson activities" you will get a lot of ideas. I open up heaps of tabs, and in a way brainstorm possible lesson activities by copying and pasting any ideas i like into a Powerpoint. From there, i think about the ones I have selected and narrow it down while also if necessary thinking about how i can adapt activities to be more writing focused

A key thing is to be a bit creative- to look at someone else's idea and think of how to adapt it to make it work for you

A great textbook i found for writing that you may want to glance at is Cambridge Writing Extra. It steers away from academic writing (which is rare) and focuses a little more on other forms. It's a little bit expensive so maybe see if you can check a preview online first or look at it in person in a store. I want to buy the whole series though as while I don't use the lessons exactly, they do inspire a direction in which to create a lesson.

I must admit I only skimmed the above posts, but I have found the most effective way to teach writing (any level, native, non-native, any age) is by teaching the writers to ask questions and then answer them. What is your topic? Who is your audience and what to they need to know about it? Why is it important? Build an outline with these questions, answer them, remove the questions and smooth out the transitions and you have your essay! You can of course simplify these questions to your students' level.

One fun game I do with low level students is to divide them into groups of 6 and give them a blank paper. I then ask them to answer the questions: Who? What? When? Where? How? Why? The first person answers the first question and folds the page over backwards, hiding the answer. The next person answers the 2nd question and so on. You take their papers and unfold them, then add in whatever is necessary to make the story (or if your students are higher level, make them do it). My favorite ever story read: Big Bang is swimming in hell this Saturday with hot dogs because it is sooooo hot! (This was during a heat wave).