So I guess I'll just put this here...
As a current year 10 at JMSS who did the entrance exam in 2018 here's my two cents on what to study for the entrance exam. Note that you really don't need to study a bunch of VCE maths and science to do well - the exam is comparatively not that hard and based a lot more on intuition. I would say the two main things you should study to get ahead is how to write a prac report and year 10 maths.
Scientific Reasoning
This component is mostly based on logic and understanding graphs and information. It isn't material heavy in terms of science, it is more so logic style questions based in science topics. There isn't much you can do to study for this. You can do some practice scientific reasoning tests but there isn't much else.
Numerical Reasoning
Again this isn't too material heavy, more so understanding patterns that incidentally involve numbers. Not much you can study as it is a lot more intuition but you can practice logic and patterns in general.
Mathematics
Probably the hardest part of the exam. This one is actually reliant on the secondary curriculum up to year 10 and is basically just maths questions. Make sure you know topics like fractions, algebra, trig, linear, probability and quadratics as that will make a difference in how well you perform here.
Scientific Writing
Might be different now, but for me this had two parts: an information response and a prac report. For the information response (not sure what else to call it) you have a paper with information about something and then have to do a write-up about said information and also include some facts about it. The prac report is pretty standard - you have an experiment and have to write about it. For study I might practice writing under timed conditions (fun right!) and also understanding and responding to information. The main thing, though, is definitely know how to write a prac report including aim, hypothesis, method, results, discussion and conclusion as well as understanding things like independent/dependent variables, potential errors and changes to the experiment.
The exam is actually not the hardest part of getting into JMSS, the interviews would be. Some tips: in group activities you should just be a good group member that cooperates and speaks, build a portfolio of extra-curriculars and outside of school activities (they care about you as a person) and in the interview itself talk about your passion for science and maths and about why it interests you. Note there may be some weird curve ball question like "How will science help Melbourne's population growth by 2050?" You probably have a genuine passion for science and maths as that is why you are applying for JMSS and that is the main thing they care about in the interviews.
Good luck to everyone trying out!