Perhaps you could lead an assembly that introduces the task using a stimulus such as an object/a letter/a scenario/a location in your school. The classes then have to mind map, plan and write their stories about the said object/letter/scenario/location etc back in their classrooms. Plan to have the whole day as a writing day! Perhaps to continue the theme for the whole day, get the children to create some art work to go with their stories. It's essentially a Big Write but that is led and facilitated by yourself without any prior knowledge by the Teachers...! Us teachers need to be kept on our toes...!! Just an idea - not sure if it's helpful!