During your school break you should honestly just relax since it's the only
real break you're gonna get from here on out until you've completed your HSC!! Burnout is dangerous! And you've got the entire year to study for HSC chemistry anyway, so why now? Go out there and have a little fun!
Luckily for you!! According to my tutor, you can definitely come back just fine from flunking prelim chemistry, because the HSC really doesn't include much of it (aside from the obvious fundamentals, like knowing your nomenclature and how bonds work, periodicity etc) and it's really more centred specifically on the HSC chemistry course. If you're missing vital fundamentals, you can flip through a Yr11 chem textbook, but honestly you'll figure it out as you go through the HSC course so I'd recommend just getting a headstart on that instead!
If you have the time and energy for it, try attaining a copy of a Yr12 chem textbook (the one you believe you'll be using) and start reading and writing notes!
If you're a little pressed on time or struggle digesting large amounts of information at once, you can also just watch videos online about each topic to get a compressed and digestible version of the same content.
Try doing a lot of practice questions & practice papers as well and you'll quickly learn what your weak spots are. Textbooks tend to have practice questions for each chapter/module, so make sure to go through those each time you complete one! And don't forget to note down all your mistakes and errors.
Other options are making flashcard decks, or teaching a friend/a toy the content you've just learned! Writing your own comprehensive study notes with the intent of posting them (maybe even on BoS) is also helpful, and who knows, you might even help someone else!
I have a bit of a peculiar brain, so I tend to work best when I'm going at my own pace and letting class-time be my revision which is why I usually study ahead of my class, and that will probably help you too! Try studying for your next lesson the night before, and then write notes on what you've learned after your lesson. Some people do daily notes for each day's subjects, others do weekly/fortnightly notes. I prefer the latter to be honest, because it requires you to exercise your memory a bit more! Personally, I write my notes for all the content I've learned for my subjects that week, and then I rewrite all my notes in the term holidays.
Study schedules and strategies are very person-specific, so I recommend trying a couple as a sort of test-run during your holidays, and then you'll figure out what helps the information stick for you or not!