the transition from yr 10 to yr 11 is probably the biggest you'll experience
if ya do extension classes then get used to attending early classes (mine's at 7:30 am) or staying behind (mine's to 4:30 pm)
also you'll be suprised at the ppl you thought weren't so brilliant just suddenly surpassing you in marks
but enjoy the most out of it before your '11 HSC!
oh and the number of free periods - either u'll procrastinate and chat with ur friends, wag school or actually do some work
p.s. i would highly suggest changing teachers (i.e. timetable lines) if u've got a shitty one because they'll be urs for the HSC unless ur good to self study