I myself do not have experience with specifically the Penrith interview, but you want to present yourself as best as you can.
Make sure a couple of things:
1) Impeccable school uniform - Ask your main office or a teacher - if your school doesn't have anything fancy like a blazer in their main uniform, sometimes they lend out special blazers for important events. BTW try not to mention that it is to leave the school, probably will de-incentivise them from giving you the blazer
2) Confidence and demeanour - Number one thing in this interview. Clear voice, good posture, only sit when offered a seat. Try also practising your handshake - I know it sounds stupid, but a firm, effective handshake where you don't pull the person in too much but are still kind of close to them is very good. Practice with your parents. Your voice should always be clear and concise, and ALWAYS have eye contact with them.
3) Questions - A lot of the time, they have some basic pre-prepared questions. You want to make sure that you prep for some basic ones, i.e. why do you think we should accept you, what are your interests, favourite subject, how do you display some values (Just in case it comes up, their motto is Altiora Peto, which means "I seek higher things" or "Higher things"), and what are some weaknesses. For your weaknesses, you need to come up with honest issues with yourself beforehand - they like some honesty in these interviews. Something that is kinda bad, but not very bad. Also, say after that you will aim to fix those things. Before answering, take about a 2-3 second pause and use things like aahhh or well to tell them you are thinking about the responses, and they aren't just pre-prepared from a sheet of paper. While it is true that you may not get asked a lot of those questions, still thinking about them means that you can get a better understanding of yourself for the interview.
4) Honesty and Naturality - This is kind of against everything already said, but you want your responses to be honest, meaningful, things that you like. When preparing the questions, don't say things that you think will sound good, say things that are truthful. Also, be comfortable in these situations, ask them "How are you?" or compliment things in the room, you want to show that you are an honest person who isn't putting on a one-time show, and will always act the way you are. Looking perfect but not being comfortable with that is nowhere near as important as looking kind of perfect but acting natural.
One last thing - print out a resume just in case they ask for it - always good to be prepared!
This seems like a lot, and it is. If anything, the order of importance is 4, 1, 2, 3. But just remember - you made it this far, you have a friend with some experience. Be yourself, and I am sure that you are a wonderful person who will make a great first impression!
