Luca,
It's a combination of all facets, but school marks are not as relevant as every one thinks.
For example, good mate of mine scraped through the HSC with a TER of 50 (yes, TER, shwoing my age!), and is one of the most talented RAAF pilots I know - best hands and feet I have seen. He blitzed the aptitude testing, did very well on pilot's course and got what he wanted.
Another mate is Navy, did TAFE after leaving school at 16, got in at 25 and now flying Navy helos.
The aptitude testing is far more weighted than the school marks, from what I have seen.