I would guess the standards are higher at your school. It was like my current (or I suppose now old) school has really high english standards, whereas the school I was at previously had low standards and everyone would get 19/20 lol.
There are standards in the HSC as well (these determine the 'standard' necessary for a B6, B5 etc.). Senior markers pick these standards. So if everyone does shit, then the standard will be lower, but if there are a lot of people doing well - then the standard becomes higher.
I can't answer what it will be like in the HSC. Maybe someone else can. Obviously there's no guarantee all your class mates will get a higher mark when they got an 8 internally. It comes down to what they wrote in the HSC, compared to everyone else in the state (which I'm going to guess will be above average).