Extracurriculars are generally good. what were you thinking of doing?
You should concentrate on UMAT first, doing practice papers (which I have available if you're interested)
I was ranked in bottom 20 out of 180 at my school in Yr10.
But once I took my work seriously, and persevered I gradually improved.
Just don't give up yet.
There is a lot of content in HSC course that can be tested, so it is less likely they'll test Prelim content.
Having said that, it's a good idea to make sure you have a thorough understanding of Prelim course.
Use plenty of pictures, diagrams, tables etc if you're using slide presentation.
Make sure you learn your speech well so you deliver it with confidence (not always looking at palm cards).
Practice.
You can ask your teacher for help.
If they're busy, maybe a one-to-one tutor would be best (tailored to your individuals needs, and more personalised feedback)