for year 12:
2 unit maths youll need a fair bit of junior math knowledge. you may be able to keep after previously bludging, but youll find alot of the time youll be in a "what? where did that come from?...oh were meant to use that thing..." situation.
english, imo, was basically just about being able to express yourself, which is something you kind of just pick up and cant really learn directly from junior school. read books. it makes you gansta.
on the odd occasion during biology you would need to remember something from back in year 11, but nothing that 10 minutes worth of catch-up reading wont be able to cover.
software design was basically a repeat of what you learnt in year 11 with a few minor additions.
for history and religion: you could have spent the last 12 years worth of your school life in a series of temporary commas, that tend to only kick in during these two classes and still not be at any disadvantage.