Basically the only issue is that if you go to a bad school, and you are currently ranked 1st internally by a lot, then you managed to screw up ur hsc test. In this case you are essentially getting purely your marks. I.e. 2nd ranked gets a similar mark to what you got, so you can only get that for your hsc mark, which means your atar will be affected greatly.
While it depends on how everyone else goes in the test, in a high ranked school you can safely assume that you will get a decent mark regardless of how badly you do. i.e. ranked 1st internally, maybe you dropped to a mark of rank 30 externally, then you can assume that rank 2,3... can carry your mark up. You might be able to get a better mark due to you messing up in the hsc than compared to if you were in a lower ranked school.