As per mentioned, it really depends on what the organizers are looking for. who are the judges? if they are teachers from your school, you have to kind of find out what they like and what they look out for.
As for 'ideas', when you mentioned reflection, I thought of a mono colour shot of maybe a car window reflecting something. or perhaps just any nice object or scenery reflected through anything reflective i.e. water, car window, mirrow, glass. there is a chance that something seen thru reflection could give you some nice 'effects'