Im starting my HSC now.
Trust me when i finished the SC, having that break was effing fantastic, cherish it, because it'll never be that easy again.
My school wouldn't let us sign out unless we had a job, but half of the year lied and got out.
If you start studying now for Prelims, you will probably burn yourself out. Once you start yr12 that wasted holiday on studying will be in the back of your mind taunting you for wasteing the little bit of free time you will have had left, depending on your subjects. Prelim & HSC courses, are friggen exhausting, and i've only been doing yr 12 for 4 weeks.
Relax, cause you'll need motivation alot later, and if you don't relax now, there isn't going to be time later to refresh yourself.