Brush up on your maths as hawkrider said, flick thru your textbook and make sure you're confident in all the topics, do the harder questions at the end of each exercise as well.
Try to improve your essay writing skills and also chem & phys knowledge
What's your school rank? If you really care about your atar then try to leave, otherwise it's not the end of the world. I know people who have gotten band 6 in standard, and you can get over 99 with it by hard work.