Okay, let me explain :3
Most kids over here start school when they're five. Some start when they're four, usually five, though. The first grade is called Kindergarten. Then - in New South Wales, anyway, where I live [I'm pretty sure in some states the year you graduate from primary school is different, like Queensland] -, at the end of Year Six - most people are usually 12-13 when they finish Year Six -, you finish primary school. Then you start high school and you take your High School Certificate when you're in Year 10. Then you can choose to go on to Year 11 and 12 and take your Higher School Certificate, which will give you admission into university depending on how well you do, or you can enter the work force. :3 Most people go to Year 12, though.
[If you fail your first HSC, I think you repeat Year 10 o_o]