The belief that HSC english is developing ideas is misguided. It is about interpreting texts and ideas. I doubt a question has ever been asked on the english HSC that went along the lines of "According to a freudian/modernist/existentialist/whateverist interpretation of some text do blah", it will also be your interpretation, which can be whatever you want as long as you back it up. Therefore, it is essentially very difficult for someone who knows a text and has thought about a text, and they have had a year to do so, to not do well. Communication is really the only intermediate skill here, and this can be remedied by practice.
tl;dr You do poorly in english if you are bad at it.