My recommendation would be to go and speak to somebody at your school who coordinates the VCE program. Or your year level coordinator... not sure what they call it at your school but it's a good place to start.
I would also frame your discussion positively and not go in there guns blazing with 'i have a right to do it, you're in breach of VCAA rules' approach. I'd ask again the reason for not allowing you to enrol in it and then perhaps get your parents to contact the school.
If you can convince them why you think doing that subject is important to your future aspirations, you're more likely to succeed in the change.
I get their concern to be honest and perhaps they are trying to warn you off doing the subject for your own good. But in the end, as long as its running at your school, it can fit in your timetable or accommodations can be made so that it fits, and your enrolment wouldn't breach any VCAA guidelines, a recommendation is as far as a school can go.