You have trials for GAT? Woah, we certainly do not do those! 
I actually thought that my school was one of the only schools that did this. However, one of the English teachers at my school (who is a VCAA assessor) said it's very common and that she is currently marking trial GAT essays for two other schools at the moment. :S
I'm not sure if it's worth it, to be honest. I guess it's good to expose yourself to the format of the test before sitting it, but that should probably be the responsibility of the student, rather than the school forcing everyone to do it. Although, if they didn't do this at my school (aka apathy central), the SAC marks would essentially get moderated down to incredibly low levels.