This is what happen to me this year in ITV HSC exam. I think I would be lucky to get 85%+ in the exam. I'm afraid to tell you that there is no 'back-up' system for ITV as ranks are not submitted, its only your mark in the trials that they look at. So a person who was ranked last in teh school for this subject can still come first if he/she performs well in the exam. What what my teachers told me, its just entirely your exam mark in the exam that counts as your final HSC mark. So if you under-performed... badluck, and just hope that the state did bad....
But if you did extremely bad, lets say a person who got 90 at school and gets like 60 in the HSC, my teacher said that they would call the school and discuss the issue with them or they would then refer to the trial mark that was submitted.
Refer to page 3 of this doc. http://www.boardofstudies.nsw.edu.au/syllabus_hsc/pdf_doc/info-technology-assessment-reporting.pdf
Its just the external hsc exam mark!