I think it just depends on the school. I don't think Reddam would just terminate your position because they already accepted you, but your student number and ID etc. will still be the same when you transfer schools, so if you want a crappy mark on your SC then go ahead, but that's the mark...
You can join in year 12, there's a year 12 guy in my group. I just think people in year 12 wouldn't be bothered to give up 2hrs of their time each week for it :P
I'm moving from an all girls catholic school to a selective as well. I have no idea what it's like in an all girls selective school though. Hopefully it's not as bitchy.
I did it last year, found the math classes to be quite disappointing, i think it was because of our teacher. Science was good though, i only did it in the last term which was a shame. My class size was small - about 10 people.