Thats complete nonsence.
your ATAR. is determined from your year 12 examinations, and assessment tasks. If you do poorly in year 11, and your School Cert. oh well. Its best to have higher marks in the early stages, so that it can build the foundations for you to gain a high ATAR. when it comes to do your year 12, and HSC examinations
Your school cert means nothing, unless you drop out. but even then now you can only drop out when your 17.