Once you've made it in the school of your choice, you'll most likely get to play in the school's orchestra. Most conservatories will have placement auditions to determine who plays what. For this type of audition, you will have to perform one piece of your choice (usually they stop you after the exposition) and a short list of standard excerpts. In that case, since not everyone is at the same level, it's usually the most advanced students who get the big parts because the jury will be looking for someone who can deliver the parts.
At this stage, you are still developing your skills. This a good chance for you to learn the standard orchestral repertoire. The excerpts you are learning now will come back many times in the future and you will be practicing them for the next few years. Learn them well and try to play them a bit better each time you have to perform them.
When you practice, pay attention to all the little details in the parts. Practice with metronome and record yourself if you can. You can also play for your friends in a little mock audition. Write yourself pointers in your music as much as you want. It'll help you focus on the music once in the audition room. Make sure you don't over practice on the day of the audition so you can perform with fresh chops.
Once the results are out, don't complain about your part assignment. It's no use and you'll only get in trouble. Keep working hard and always do your best on the parts you've been assigned. Don't worry about not getting the parts you want. You turn will come!
I am associate principal horn of the Singapore Symphony Orchestra and the author of the progressive methods. I'm happy to share my experience as a horn player and teacher with you.