finish school!!! and get the best mark you can. when i went for an interview for cadet program recently those on the panel were very concious of my UAI, they knew it to the .05 without even looking it up. a good mark at school isn't everything, but it's another tick against your name.
work casual jobs as well if you can cos when you're young (i am too!) and go for an interview where they ask you basically all behavioural questions you need to have lived and worked a little in the world outside of school to have an ok source of answers.