Auditions

I'm not someone with experience of such things, but here is what common sense suggests to me: you are auditioning to be a student, which means the judges do not expect a 'perfect' performance. They are looking for someone who shows some basic musical/technical competency, and who seems eager to learn. I would guess that presenting an intelligent, expressive performance is as important as demonstrating what your fingers can do. So focus on the music, and don''t worry too much if small errors occur from time to time. Be relaxed! Easier said than done, I know - as preparation, perform your pieces as often as you can, for as many people as are willing to listen. (Don't hesitate to upload something here, too!) Finally, bear in mind that music schools (like other educational institutions) NEED students: the judges are looking for reasons to accept you, not to turn you away! Best of luck with your audition!

David Krupka's advice is excellent. As a nurse and someone who suffers from performance anxiety (PA), I'll add some background as to what's happening, and a couple of additional suggestions. PA is caused by a rush of adrenaline/endorphins produced by your nervous system's sympathetic response ("fight or flight"). Ideally, you want to shift your body's response from a sympathetic response to parasympathetic ("rest and digest"). You can test which system is in play by taking your resting heart rate, then taking a deep breath, holding it in, and seeing how high your heart rate jumps. When you release the breath, if your heart rate stays high instead of coming back down, the parasympathetic system isn't working as intended. There are some possible reasons for this:

• additional stress (the most obvious one being fear of the upcoming audition)
• Lack of sleep / fatigue
• caffeine / sugar intake

For stress related to PA (i.e., fear of auditioning), it's helpful to identify why you feel that way. For me, the fear of being underprepared or making mistakes is huge.  I combat this by, as Mr. Krupka suggested, playing it in front of others who are not there to judge me (or haven't paid to see me). A good rule of thumb is if I can play it 3-5 times in a row without making mistakes, then I'm good.

During the actual audition, it's helpful to think of others in the room as just people who want to hear you play, instead of people who hold power over you. As a classical guitarist, you've already learned (or should have) how to relax your body from the top down, relaxing each muscle group as you go, to ensure there is no tension in your hands when you begin. 

On the day of the audition:

• "Dress for success." It seems like a minor thing, but when you dress well, you just feel better about yourself. 
• Limit your sugar and caffeine intake. The physical response they cause in your body is similar to how it responds when you are stressed.
• No pot, alcohol, or other illicit drugs. Seriously. Not today. I'm not saying a little (legal-in-your-state) smoke would ruin your performance, and for some it actually might help, but this is not the time to test it out. Ditto for tranquilizers; they get you *too* relaxed and your performance will go out the window.
• Take a breath in, then exhale slowly. Slow your breathing as you relax.
• Think of these people as friends. They want you to succeed because they want you in their school! :)
• Positive thoughts, ONLY! Don't focus on the negative, or what could go wrong, but on the positive. Again: They're all rooting for you! In your mind, visualize your successful performance. It sounds like new-age nonsense, but it works. 
• Smile at your new friends and enjoy yourself!
• ...but don't lose focus on the music. I went blank once during a performance because I was too "into the music" and forgot where I was!

Things to do now:

• Start a routine of regular exercise, meditation, or yoga. Yes, really. Although they work in different ways (this post is already long enough!) all of these things provide your parasympathetic system with practice, which will produce a lower resting heart rate and lower the sympathetic response (which means less adrenaline coursing through your body, which leads to trembling hands). That said, don't go big right now! Small changes will lead bigger gains in the long-term.
• Practice Controlled Breathing for a few minutes every day. The Medical University of South Carolina has an excellent, simple-to-follow guide (I don't know if links are allowed here, so I don't want to chance it).

I hope this helps. I believe in you! You've got this.

There was an excellent session on performance anxiety by Aija Reke here on Tonebase:

https://app.tonebase.co/guitar/live/player/vln-aija-reke-stage-fright

Some great suggestions above.

At one time when I was preparing for one of my first "big" performances I did a pretend performance everyday for 2 weeks. Did all the things I would do on the day (warm up etc). The first few days there were some issues but after about 3-4 pretend performance I started to relax. The pretend performances were conducted in different places around the house and outside, often on different chairs and sometimes I grabbed someone to be an audience or I would record myself to create some pressure. On the day it didn't seem such a big deal as I had been more or less rehearsing the same moment everyday for 2 weeks. I focused on the music and musicality and played really well.