Ben,
As previously mentioned don't let anyone tell you, you can't achieve something. I'm no vet but I'm in my final year of an engineering degree. I've worked full time while studying part time for the past few years. I was a very average student at school but while I've been at uni I've maintained pretty high grades, this is because with school you have to be there, UNI you need to want to be there to achieve a greater goal so it makes it easier to understand the work because you're interested in it. Also keep in mind that after 21 (i think) you're classed as a mature age student and you HSC marks aren't overly important. They weren't for me. If you can do a tafe course once you finish school it will be a huge help in getting into uni. That's basically what I did.
Anyway, theres more then one way to skin a cat as they say. If it's what you want to do you'll get there.
Tim