Once you have your HSC they cant take it away from you. But if they haven't submitted your school results to the Board of Studies then you could be in some sort of bad situation.
The Board of Studies has its rules regarding assessments and elegibility for the HSC. However, the Board of Studies within its rule book states that a candidate must follow the regulations of the institution that they are doing their HSC (School, TAFE etc). One would assume that statement gives the school the right to withhold the marks if they have a valid reason (and in a public school / TAFE the reason would have to be very very very good as these institutions must follow the guidance of the NSW Dept. of Education where as private schools don't).
I have heard that withholding HSC results happens mainly to students in private schools, not so much in public schools.
The time for N-Awarding in specific subjects has passed (unless you have got your two warning letters already).
At my school, one of the teachers has threatened to put Year 12's on class attendance cards (normally given to junior students) for not turning up in the last weeks after the trials. Its pretty much a waste of time, she cant do much even if the student doesn't turn up.