Hi Hana,
when I tried out for JMSS (2010), I went to the weekly exam prep classes at James An College. While they were more directed at the Melb High/Macrob entrance exams, since JMSS was only on its first year then and there wasn't many resources for them especially, I did find that James An helped me with time management and developing exam skills. On top of that, I did some work at home. I read some books on general science topics, and also practiced analytical/expository essay writing essays at home with science opinions/debates I found in the newspaper. And as math a weakness of mine, I worked through a yr 10 math book at home. I also attended the analytical writing workshop at Hendersons and a mock exam there too which helped greatly. I definitely recommend Hendersons, it's pricey but for me it was worth it. While this sounds like a lot of work (I was pretty desperate to get out of my old school lol) I'm pretty sure half the people at school didn't go through this much effort. It paid off for me though, cause I got to skip the interview process, but a lot of people I know got in with an interview having not done as well on the test- because they were able to demonstrate an interest in science-so don't panic just yet! Some people get last minute calls for interviews too! Now in the unfortunate case that you don't end up getting in though, take it as an experience. I did terrible on the Macrob entrance exam, didn't prep at all, so I took that major failure as a wake up call and turned those averages into superiors within a year. If you can take that disappointment and turn it into drive and motivation to prove yourself, there's nothing stopping you from dominating the competition next year! Best of luck!
And on the note about other schools, since coming to a selective one I've realised I probably could have gotten the same grades at my old school, and with more confidence throughout the year. Selective schools really kill your motivation and self-belief in Yr 12 -_-