12 hours of writing is the equivalent of 24 hours of oral. Depending on where you are (hangzhou, if I remember) that should be about 8 to 14 thousand. Seriously, Chinese university graduates have writing skills that compare poorly to high school students in western countries. Working in a foreign language and trying to navigate our foreign conventions only makes it harder for them to express their thoughts coherently. Marking (or editing) their work is very time consuming and often quite exasperating. It's also exasperating for them, especially when they're trying to learn something that just never quite jives with what they've spent a lifetime doing. You've got to walk a tightrope, cultivating a capacity for hard-nosed cirtical thinking and editing, and yet instilling a sense of confidence and self-worth. Lots of luck with that.
Keep your introductions as short and to the point as possible, give them as much in class writing as possible, and mark as much in class as possible. Teach them to bubble plan, to flowchart, to separate ideas into distinct sentences and paragraphs, and how to write a topic sentence. Teach them how to KISS. (Keep It Short and Simple. What did you think I meant? Jeez, you guys)