Sorry to hear that. Given that you have completed your HSC, you are technically no longer a student at the school that you attended, which I believe implies that you would need to re-enrol at your school (if possible). While you may still be able to catch up on content that was covered in the first term (although I assume that you would be still familiar with most of the content in the subjects that you took anyway?), it is unclear whether your school or another school would allow you to enrol at this point in time instead of having to wait for next year, which would be a highly impractical option. Because of this, it would be a good idea to contact your school/the school where you are thinking of repeating year 12 to confirm this.
On a similar note, if you are determined to repeat year 12, you may be able to do so through
TAFE. From what I have observed, it appears that there is no particular starting time for this program, meaning that you may potentially not have to worry about making a slightly later start than your peers. Additionally, TAFE offers a multitude of subjects, so it is likely that you will be able to retain the subject combination that you had when you completed your HSC.
With that being said, are you 100% certain that repeating year 12 is the best course of action? While your HSC results and/or ATAR may have not been as high as you would have hoped them to be, keep in mind that repeating year 12 will cost you an extra year, not to mention the possibility of experiencing the same situation that you are currently in. During that year, you could instead enrol in a university degree (or similar) with a lower ATAR requirement. If you perform well in your first year of university, you could subsequently attempt a transfer to your preferred degree(s).
Ultimately, the decision is yours to make, and I wish you the very best of luck, irrespective of the path that you choose!
I hope this helps!
