some time in assembly will definitely help, but you should remember that the exam is only in front of one or two examiners, and not a hall full of people
how do you usually do your performance assessments? from year10, my teacher made us perform in front of her, and her only... it was really intimidating the first few times but you get used to it.
nerves happen to everybody. i know it's easy to say and hard to do, but try to focus on the emotions of your song - that's what i did when i performed, and i tried to forget about the examiners. i also took some deep breaths before i sat down at the piano and yeah...
hahah i was so pissed off last year, my teacher gave me a DAY'S notice that i was performing in assembly o__O i remember making a thread in this forum asking for advice... *reminisces* i ended up doing pretty well, only stuffed up the hard part a tiny bit.... so yeah.
to answer the initial question, unless your performance pieces aren't up to scratch i don't think it's necessary for hours of practise
i barely practised for a few hours each week... then again i was hardly home
i think you should focus a lot on your weaknesses, my weakness was aural and luckily we had heaps of practise in class. if you suck at aural, try and DIY at home, change your radio to some foreign channel, record it and analyse it like you do in class.
when's the hsc aural exam this year? for us, it was on the first day
ahhh well, better to get it over and done with