$15000?! Whoa, I only paid $105 or so per semester, but I'm on centrelink
Work load is a lot less compared to the work I had in Year 10 (private school). Most of it is optional really. So in the science and math subjects, you get a course plan at the start of the year with all the exercises you should do, you don't have to do them cause the teacher doesn't check. In chemistry, I don't think we ever had an assignment in Year 12, only tests, pracs, don't think we even had to do lab reports, but my memory's a little fuzzy.
But it's really up to you, most teachers don't push you like they do in high school, but if you aim high, then they're there to help. Some do, but they can't force you like in high school (by that I mean you don't get in trouble if you don't do the work). You're treated like adults. Also, my biology teacher once said that the assignments/SACs at RMIT (science and math) are easier than the ones at other high schools, because they have to cater to different people (e.g. adults who've been out of study for a while), so not sure how that affects study scores/ATARs