haha, are you serious,
if you can't hit a 90 in trials, there is now where your ranks would increase, this would prevent you from getting a band 5, because say you 98 in external, than since your 11, you will get 11th rank mark as you assessment mark, say the 11th gets person( not you) gets 80, then you hsc mark would be 98+80/2= 89 ( which means, you are likely to get a band 6, if you score the highest external mark, say 98, the pray that the 11th mark is above 80) This would get you a band 6.
Like if bio for example, I am ranked 1st, if I get 98 in my HSC, this would be HSC mark ( 98+98/2=98) I could easily state rank. Your HSC mark is the average of you assessment + HSC exam/2= ?
Good luck, even if you move a rank of asay about 2 up from 11 to 9, then score highly in external, the 11th mark you get as you assessment could be anything. This is the reason, why you need to compete for getting top rank, like top 3 to secure you band 6.
Don't worry, anything could happen on the day of HSC exam, just pray all you class those good, so they can left you up.