First off, thank you to everyone for the generous help and advice. Noting that some of you have given me potential ideas, I also want to provide a few more details which may clarify my situation.
I attended a top 20 selective high school and after studying relentlessly for more than a year, I was able to achieve an internal ranking of first. During my time at school my essays generally scored in at least the B-range (scoring 17 & 19 in my trials - of course, I recognise that my school standards may be much lower than that of the HSC).
After the real HSC exam, I felt quite confident that this particular essay (let's call it Essay A) could score in the B range +, compared to the second essay (Essay B) which I thought was considerably worse. However, I was extremely surprised when I received my raw marks, which revealed I had scored a raw mark of 7/20 (D-range) for essay A and 18/20 for essay B. After 2-3 days, I finally decided to order my examination response which was sent to me yesterday. I have since given essay A to a few friends to read who where thoroughly surprised I had received only 7/20. Believing my friends were just being nice, I also posted my essay on the economics section of this forum, where two fellow members suggested a rough estimate of between 14-17/20 (I do recognise these are simply estimates since there is no marking guideline available nor are they HSC markers, but that truly is a significant disparity).
I am currently considering this option, having sent my essay to my experienced head teacher to read over. Thank you for your suggestion!
Again, I'd like to thank everyone for your help and time.