all of the previous answers are true. however chances are you haven't been reading fiction books every month since you were a child. so here's how you exploit the hsc creating writing marking scheme:
have a central metaphor/gimmick that runs through your story. this will need to be based on whatever theme you choose for your story, and it will need to be very obvious. start your story off with the gimmick, then move onto narrative, then bring it back to the gimmick in the middle, then go onto narrative, and then tie it back to the gimmick. the gimmick can be anything from a biblical allusion, an object, a culturally significant thing (they love stories about different cultures), a myth/legend. basically anything. however for it to work, you have to choose an explicit theme e.g. love, family, death, power/corruption. this will typically get you into 17+/20 territory if you do it well.