If you've done a total of 40% of the assessment marks for your subject, that means you still have 60% left to be assessed. So far, you're only 5 marks from 1st position. I'm keen to wager, that the first couple of assessments at your school weren't too difficult. There's still 60% of your marks to be determined, just work hard. Don't stress about the little things, just do the best you can. As, 'Theone111' stated, the biggest discriminator is the trials (and also the half yearlies [provided your school examines with a half yearly exam], as Trials are the hardest and test students replicating the HSC. Don't lose hope, just try your hardest. There's still plenty marks left to be determined, and with a little good luck you might be able to improve on your ranks.
PS. Congratulations on your marks thus far, you should be really happy and proud for what you have already achieved. n_n