moving to america!

Where are you moving to, specifically, allin? Here in the USA each of the 50 states has its own rules on these matters. Generally, a state Board of Education makes specific rules on what credits are acceptable as proof of work completed at a particular grade level. You might find that your work in Australia entitles you to enter Grade 12 in September (or whenever school starts in that state; in Hawaii,for example, the school year often starts much earlier, like mid-August). But, again, it all depends on where you are. So without more precise information, none of us can really help you.