Not only is it the ranking but is the relative gaps between marks. For e.g. a person coming 5th by 2 mark will be considered differently to a person coming 5th by 10 marks.
Bear in mind that those essays are coming up to being 10 years old so the course requirements and expectations might be very different to what is needed for this year's HSC.
+1, they should definitely be giving out raw marks/weighted marks. In terms of actual exam papers that would be a significant cost in sorting and out posting them out..
What needs to be said has already been but just to re-iterate no one is guaranteed/excluded an ATAR. This means someone from a 500 ranked school get an ATAR of 99.95 and a person from a top 10 school can certainly get a very low ATAR. But from averages this 'likelihood' is very much dependent on...
School rank will make all the difference. But as a random guess, I'd say around 80, you really need to perform strongly given the low scaling of some of your subjects. Good luck!