The main thing you have to do in a creative piece is have flair and originality. Its great to have a good idea, but if you have a mediocre one you can compensate for that with form.
Innovative form always goes down well with markers. Like a regressive narrative (where you write it all backwards), eclectic POMO text types (Journal entries, medical reports) do exceptionally well. Split narrative perhaps??? Just make sure that your form always mirrors your purpose. EG if you're writing something sad and lamenting then don’t write it in limerick form.
And for your ideas, be specific. If you love writing about, let’s say, death, then focus on the transience of life that lead to death. Change the form; add a doctor’s report in there, a diary entry with a poem in it etc... Use semiotics (symbols) of death throughout. Add language devices; metaphors, a motif, like sunset.
Add a quote if you like. If you’re doing on death and sunset motif then use one like:
"Sunset exalts its known symbols of transience" - Gwen Harwood.
Dont let teachers tell you what you can’t write about. Dazzle them with your execution and your brilliant use of form. Have fun with it, it’s a chance to be creative.