That sucks that you only have one folio - I had three when I was at school 2 years ago.
I would recommend something you are genuinely interested in. Write down on a piece of paper what your 3 top hobbies are.
For me this was
1. Hockey/sport
2. xbox/computer games
3. drawing itself
So my folio ideas were - hockey stick, a fore-runner spaceship, and some stationery, like a new pen or a mechanical pencil etc
Also - another approach is to find out what you mindlessly doodle - when you are bored in class, when on a phone call, what exactly is it that you are sketching?? For me this was wepaons - swords, guns, space-ships, some randon scribbles of spikes - something that looked like crustacean shells etc etc
I did consider designing a sword for a RPG, or a book cover about some medieval trash story - based on that .
Finally, another good approach, is to ask yourself, if you were to become a designer of anything what would it be? Me? - this would have to be graphic designer for logos, or a product designer for input devices. So - I would be crazy keen about perhaps designing the next Rat mouse, or a new razer keyboard or a logo for new gamin perihperals company
My actual folios were
1. water bottle
2. space ship (game)
3. logo for an air-force organisation (game)