I actually have no idea how they mark the exams.. Like I was almost unaware until after the exam had been completed ;-;
They compare your results against everyone else who sat the exam, which reflects upon the score you will receive later in August.
Your score from each test will be put into one of the six categories: below average (bottom 20%), low average, average, high average, above average and superior (top 12%).
Eg; your score for maths is 30/60, which places you in the superior category for maths, as only the top 12% of the population obtained that score.
Generally, to get a place at any of the schools, you'd need to get 
at least a high average for a few tests, and about one superior at the least. 
However, your current school affects how likely you are to get an offer, if the 5% rule applies.
Eg; you go to a school with 200 students in your year level and 12 people are trying out for the schools. Even if you performed really well on the day (but not as great as everyone else who tried out from your school) you might miss out on a place as only 10 people (5%) from your school can be accepted. 
Essentially the marking system is just based on how you performed compared to everyone else.
Hope this helps… 

Do you think they mark one piece together or separately?
Probably separately, but they're likely to cross-check with each other
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