My friend who is 0.1% ahead of me is 5th, and he got high 60's in trials. He is pretty bad in exams.
You should be fine as long as other students above him do well too.
And any way, there is nothing you can really do at this point in time other than help others in your class study and do well (this will prevent you from losing marks as the higher marks they get then the higher the assessment mark you can possibly get). However, do keep in mind that you also need to study so don't help them 24/7. You need to make sure you personally do well in the final HSC exams too!
Other than that there isn't much you can do so worrying about it will get you nowhere. But honestly you shouldn't be worried, its not too bad.
Just maybe take a bit of time to help them in that subject and make sure that they do well in exams and you should be sweet. Again though, don't waste too much time on them as they should be doing their own work too.