Correct me if I'm wrong but....
If you got 12-13/15s and 16/20s in the HSC on average, you'd be hitting a 90 exam mark around (89-91) - give or take.
In fact, due to option aligning for some texts in the HSC - you could only need 14-15/20s. E.g. Hamlet Mod B - a 14-15/20 can be compared to a 16-17 in another text for Mod B.
Being ranked 4th in Advanced English (for a school in the 250s) --> statistically, you probably won't get a band 6 HSC mark BUT school rank fluctuates anyways and cohort performance changes from year to year so who knows.
Your trend is a positive one but let's face it - your internal MARKS =/= accurate indicator of how well you will do in your final exam.
Factors to consider for a band 6 HSC mark:
1. On the day of the HSC exam. You might bomb out or you might be on god mode. Things can happen on the day - you can get an exam mark that is above, below or equal to your assessment mark.
2. Your school. You might being marked too easy or too hard. School marking standards might differ from HSC marking standards. Dodgy marking. List goes on. You could get 18/20 for an essay during TRIALS then get 12/20 for the same essay during the HSC cause your school marks too easy.
3. Your peers and your internal rank. Imagine you got a 90 exam mark but got a 88 assessment mark (because someone/people ranked higher than you during school stuffed up their HSC exam). I know someone who got a 90 exam mark but a 82 assessment mark for Advanced English --> final HSC mark of 86.
Can you get 90? Yeah ofc.
Will you get 90? Depends on you and your cohort.
To be safe, you'd want to be ranked even higher.