A quote from iamtom:
I know several people who entered in year 10. We're not privy into what you need to study - or the entry criteria - however I can make a generalisation!
For the most part, you need to do a sport. Several of the people who entered in year 10 for my cohort now play state-level Waterpolo, or high-level cricket, etc. It's not necessary, though, but they definitely look favourably towards it. Actually, I'll contradict that and say you need to be involved in co-curricular activities - you need to be able to demonstrate what you can offer to the school. Smarts are not enough! Scouts, sports, volunteer work - this is all good in the eyes of our school.
In terms of academics (I'm pretty sure you have to sit a test as well as the interview) English is definitely looked at as more important than mathematic capability; when I entered in year 9, I was in the top 1% of the people who sat the tests in English, but I was in the bottom of all entrants for Maths. I got straight in, not even on 'principal's discretion'. Alternatively, my brother failed to get into MHS this year and he was in the top 1% for maths and in the lower-middle for English. So work on your English! Essay writing is very important to get in.
If you fail to get into MHS for year 10, you can enter in in year 11 and even year 12 is possible - I wish you luck!