Totally agree with finestkind, I put in my application first term in Year 11. It gave me plenty of time to sort out paperwork and medicals before having to juggle it with the stress of year 12. If you don't have any hang-ups on medical or missing/incomplete paperwork then your probably a minority. I also just realised that I applied before 16 yrs 6 months of age, obviously no problem there.
My grades are not exactly spectacular (B's, B+'s at the best of times). Having better grades would likely make you a more competetive candidate althoug keep in mind it's definately not the only thing you're asessed on. My advice is simply to put in your best at school and show to the recruiters that you've got the ablility the work hard and get results (improvement in grades).
Best of Luck
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numbers guy- "ne1", took me a while to figure that one, might want to watch the engritch there.
The Assesment day is simple (in a DFR simple kind of way), I think you may have this mixed up with other parts of the selection process. Its comprised of a Psycological examination (having a 'little' chat with a slightly creepy bloke who sits dead upright and has freaky eyes), Brief Medical Examination (Height, Weight, BMI, Urine test, eyesight, hearing test and ECG), Full Body Medical Examination (be prepared to waddle like a duck in your jocks) and a Defence Interview where the questions will probably vary from person to person (If you know your stuff and are fairly confident this can actually be an interesting conversation)