Hey there, Sarah!
To answer your questions:
Would I be likely to get in, or is everyone like me/better than me?I'd say your in a good position, and should definitely apply for JMSS (because we're one of the best schools ever, and we're grounded in one of the best universities ever...
no bias). Based on those results, I'd say your good to go. In terms of your competition, there will be people who showed up to the test, doing all their research of what's going to be on the exam, etc. etc. on their drive to the test, and people who started studying for the exam fairly early (as you see on AN from a few of the questions from other users like yourself) like 10 months in advance or something. In the end a mix get selected.
How hard is the entrance exam?Hmm... well, that really depends on you. You just have to show as much knowledge as you have about the topic in your written pieces, and be quick and fluent in your multiple choice sections. If you like science and maths, you'll enjoy the exam. Just know your year 8, 9, and majority of 10 for maths and science and you'll be fine
Is there a lot I need to know before I apply?You need to know your year 8, 9, and majority of year 10 for science and maths. If you do, I'll be shocked if you don't get in.
TipsExcept for that, in the interview they are looking for passionate students not just smart. When your in there, show them your passions for science. And just be a friendly person in general. The best interviews are those that are like conversations, so aim for that.
Also, just because we're a science school, doesn't mean we don't appreciate the beauty of English. You should put your results for your English also in your resume for your interview. It's good to show that you have an aptitude for other subjects, as well. Though, being a science school, we of course favour the science orientated people because that's what we cater for!
Good job on your interest in neuroscience, genetics and biotech! ^_^
(again, I'm totally not biased here) If you come to JMSS, you might want to do bioinformatics

I've always been good at science, but I am not sure if I am good enough for JMSS.
It's not so much if your good at science, but rather if you have a genuine passion.
Good Luck!
If you have any questions, feel free to shoot me a PM, I'll be happy to help!