In my last year in high school, we won first place in our category. It felt like we won the super bowl! Great memories!
Playing in a band is a great way to learn team work skills, develop tolerance and understanding for your colleagues on top of learning to play a music instrument which takes discipline, patience and perseverance. What you learn in band will help you for the rest of your life no matter what carrier you choose. And you make lots of friends!
So here are a few things that the jury will look for in the competition. Work on these things together as a group and you'll increase you're chances of winning.
Ensemble and intonation: If your group is together and in tune, it'll sound good. If not, it'll sound like you're not playing the same piece. It's not only about playing the notes at the same time but to be exactly together with the same style, same length of notes, same dynamics, same articulations etc. To achieve that, you need to pay attention to what the section leader is doing and try to copy that. You need to breathe together, listen to each other and feel each other. It's a group effort, not the addition of many individual efforts. There is a time to blend in and a time to stick out. Be wise about it.
For intonation, each section should do some sectionals and work on each note and chord thoroughly. Ask a teacher to help you if possible. It's not wrong to use an electronic tuner for some notes but remember that in the concert, you're not playing with a machine; you're playing with human beings. It's not about who's right or who's wrong. It's about how we make it sound in tune together so don't be too stubborn about the tuner if everyone else is playing at a certain pitch.
Dynamics: One thing that I see a lot when I coach bands is that students are quite good at playing loud but not so good at playing soft. It's not that difficult to learn soft playing if you spend some time on it and it makes a great effect. Also, if you can play very soft, it'll make your loud passages sound louder because of the contrast it creates. If you can do that, you'll impress the judges and increase your chances.
Energy and musicality: It's easy to feel when everyone is into it, isn't afraid to take risks and goes for it. It immediately creates a full, energetic and moving sound. Don't just play the notes, play the music. It's the difference between a good band and a great band.
Stage presence: The show starts when you walk on stage. You don't need to be like an army regiment but you need to look good and proper. Sit up straight, smile and look confident. Wear proper clothes and PLEASE: no withe socks with black pants!!!
Have fun!: As I said many times, if you have fun playing, people will have fun listening. At the end, the reason why we play in a band is because it brings us joy. The performance will last a few minutes for which you will spend hundreds of hours rehearsing. Make sure you enjoy yourself during those hundreds of hours too. You need to take it seriously but never forget the main reason why you're doing this.