It really depends on a few factors.
How many people are in your class?
Are you really studying as hard as you could be?
Is your class/cohort a competitive one?
hopefully theres only 10 people doing phys, otherwise ur basically screwed, ur way too behind by now, u'd only be able to come middle if u come first in all the remaining exams.
The other thing is, though (and I know this really impacted a lot of people's marks when I did my HSC), what was the weighting of the half yearlies? ANd what will the weighting of trials be?
For me, some of my trials' weightings were up to 40, 45%... So there was quite a bit of room for improvement (though admittedly, my marks went downhill VERY fast between half yearlies and trials).