I go to St George TAFE myself and do the Accounting Course from 2-6pm on Tuesdays. My school is good in the sense that it ensures that the period that is missed out on in last period is a free period, which is very convenient and means we don't miss out on any of the content from any of our other subjects. However, your school may not be as willing and as accommodating or flexible as they should be.
Sometimes you may face resistance from your school, and they may do their very best to make the situation as inconvenient for you as possible. My sister, who finished the HSC in 2008, went through such a trauma trying to do the Accounting Course (and it also happened to be at St George Tafe). Her school didn't (and still doesn't) support the course. They purposely allocated maths into that timeslot so that it would dishearten my sister from doing the subject - they made it very clear that they didn't want her to do the course. My sister persisted and was adamant she wanted to do the course, and despite the very best efforts of her school to discourage her from doing it, my sister was willing to sacrifice maths time. Admittedly, she ended up dropping maths extension 1 because it became too much for her as she was missing out on those lessons (and she didn't have a tutor who could help her catch up and fill in the gaps). In the end, Accounting was my sister's TOP SCORING subject on her HSC (she got 96). She is now doing Accounting at University.
The moral of the story is BE PERSISTENT, DONT GIVE UP and whatever scenario they throw at you, DONT LET YOUR PASSION BE OVERRULED.
Good luck!