I used to have 9-12 free periods a week during the HSC. (I had dropped English Extension 1, Music 1, and 2 unit Mathematics).
Yes, you will tend to use 3/4 of the year's free periods to play connect-four/track-racer/tic-tac-toe in the back of your maths books. OR as a way to catch up on that well earned SIESTA time!
BUT - that last 1/4 of the year's free periods; USE THEM WISELY! By then, HSC exam panic may have been triggered, and you will start acting on that free-time effectively, turning it into study time. If not, then do so. Immediately. Because seriously, do you really want to remember for the rest of your life that maybe you only JUST missed out on being the world's most BRILLIANT doctor.. why? To play a little extra tic-tac-toe?
Just my two cents.