Realistically, to get a really high ATAR (90+) you need to:
a) Heavily restrict hobbies (1 or 2 MAX)
b) Heavily restrict social time
Um, no. Definitely not. You just need to be really organized.
I know that everyone manages their time differently, but this is how my own year 12 worked out. I'm not trying to do this to be like ooh look at me or whatever, but seriously, please don't be under the impression that you need to give up everything for year 12! That's not at all healthy!
-I missed 6 weeks of school due to various competitions and music things
-I was exceptionally jetlagged for some time too...
-I played in a youth orchestra which had a lot of rehearsals
-I was leader of music groups at school and was music captain
-I entered writing competitions
-I took the SATs
-I took a diploma of French
-I completed the American college applications
-Did a number of media related things
Anyway, I wouldn't recommend doing as many things (especially if they take up a LOT of time - I was overseas a lot and whenever I came home I was pretty overwhelmed), but make sure that you have time to spend with friends. Don't go overboard - if you're of the legal drinking age, don't get drunk every weekend as it'll knock you around a lot - but make sure that you can relax. I did do well in year 12 - I surprisingly got a 99.95 - but the things that matter to me from that year were the competitions that I did, the music that I played, and the friends that I had (one of my friends recently came and visited me for a day at Yale!).
One thing that may highly increase your efficiency is being aware of how you best work. How many hours of sleep do you need? When do you best sleep? When can you best concentrate? Do you do better if you study the day before an exam, or do you need the mental space? What study techniques are the most useful for you? Also schedule in your study time, so that you KNOW that in this time space you HAVE to be studying... you'll be efficient (close FB and the internet... try going to a library where everyone else is studying too).