Bio is a very content based subject, It's pretty much a chunky syllabus with a lot memory and rote learning involved. features, functions, definitions etc..
I enjoy the subject and so I find studying for it quite interesting, if you like what your reading (because that's essentially how you study for bio, by reading the textbook) then it should stay in your brain.
It requires a bit of study, depends how good your natural memory is. There's a bit of technique involved when answering Bio's short answer style exam questions questions. I find writing short answer responses in the specific way that the HSC markers are looking for harder than actually learning the actual content. Bio markers are always snobby when it comes to how you answer the question, I think that's what your teacher means when they say getting band 6 is going to be difficult.
It pretty much boils down to this; if you're interested in Bio do it, if you're not, then don't.
Good luck
