If you were first, then your assessment mark is determined by the highest HSC exam mark of anyone in your cohort. Your HSC exam mark is your own exam performance, which is not affected by anyone else.
Only you can drag yourself down if you are outright first.
Like has been stated above, if you were ranked first than your results could not have been dragged down by others and could only be the result of your score, or could even possibly have been improved by the scores of the others in your cohort.