Getting a 50 isn't all about knowledge and intelligence - you could be the best and brightest kid in all of Victoria, and still fail to get a 50 without the proper preparation. To get a 50, you can't just rely on knowledge of the subject and curriculum, you need to:
a) Work hard, and make sure you don't make lots of mistakes under pressure.
b) Be familiar with the common questions asked by VCAA.
c) Be lucky.
Whilst a and b can be side-stepped (a by being really fast and proofing your work 20093409820984309 times, and b by just being THAT good), c is another story. Someone who gets a 50 one year, may not get a 50 another year. Hell, the difference between most scores above 40 is a bit of luck depending on the day. Not to say people who score that highly didn't deserve it, but rather they could've easily scored higher or lower just based on the day/questions in the exam.
Looking for a teacher who knows EVERYTHING is stupid. A teacher that can do your exam for your is pointless if they can't teach. If, because of the teacher, you can understand the curriculum, and they have given you enough independent thinking ability to answer the questions you come across, then they've done their job spectacular. If they've done that, who honestly cares if the highest score they can get is a 50 or a 10?