Creative writing is a pretty broad topic so you need to decide if you're going to focus on a particular style (I would recommend this - poems, short stories, group stories, movies, etc) or give the kids free choice which might overwhelm them. You need to give them lots of examples and guidance before you expect them to just write.
Start with adjectives. Draw a table with 5 columns - one for each sense - and have the kids take a walk outside during the lesson (get this okayed by the KTs since they're not used to lessons not being conducted at a desk). Tell them to write down everything they can see, taste, smell, touch and hear - do each sense in stages (close your eyes; what can you hear?). In a separate lesson, have them use those adjectives to describe their school/location - this can be a spoken or written task.
Here're a few links to sites from a quick google search.:
http://www.tengrrl.com/tens/017.shtmlhttp://creativewritingprompts.com/http://www.brucevanpatter.com/funstuff.htmlhttp://www.creativejuicesbooks.com/
