For assessments, you do them gradually over the year, but for trials (i assume your school will follow the final HSC exam format) and the HSC, you just go into the exam for half an hour to 45 minutes and play your pieces, stopping after each one for the examiners (usually 3) to write notes
You write your own program and present it to the examiner on the day. If you're doing only performance, it will obviously just be 4 (for music 1) pieces one after another, if you're doing vivas as well, you can decide the order, and if it's composition, that goes in earlier, I think about now. (I'm 1 performance, 2 vivas, plus core)
Good luck