whisper I did the JMSS test this year. The math and NR part for me was easy and its relatively similar to the contents in previous Selective school tests (EDUTEST not ACER).
For science reasoning most questions were on interpreting information, for example they had given us four images of different objects on top of another and asked which fossilized object was the oldest. I do not remember much but I do know balancing chemical equations, acids and bases, and why is it hard for us to differentiate the colours in fireworks (kinda random probably do not need to revise on) were on the test for one or two questions. ALSO revise on the periodic table, because it may come in the test even though it didn't for me.
My recommendation is to look through almost all of your year 9 and 8 science textbooks and some year 10 topics. Also the writing part had given us a stimulus sheet( about organoids and stem cells, also included quotes and research on organoids) and told us to use this for our essay, and include answers for three questions; what is the difference between organoids and stem cells, what are organoids, and what are the ethical dilemmas that may arise when experimenting with organoids.
Sorry I couldn't remember much from the test, and PLEASE do not go to JMSS because your parents want you to, go there because YOU want to go. My sister got in to JMSS and my parents are putting a lot of pressure on me, but that doesn't affect my studying because I REALLY like the school.
Hope it helps!