Practice listening and get a sense for basic harmony and chord progressions (e.g. I-IV-V, ii-V-I, I-Vi-ii-V-I), know the sound and how to play major and minor scales, and definitely work on reading because as elgecko said, the basslines are likely going to be written out for you and being comfortable reading them will help. Also, practice internalizing time - work with a metronome every day, because you are going to anchor the band along with the rest of the rhythm section. You want to make sure your pulse can "lock" with them.
Other non-musical things to keep in mind that will help are:
1) be a good member of the band - don't noodle in between songs, check your volume and be sure not to be too loud (too soft, is seldom an issue with an amplifier), and be easy to work with. It is never to early to develop professionalism!
2) It is a high school band, which means, everyone there is a novice, no matter how highly they think of themselves. If you have humility, listen to the direction from the band director, and ask questions when you need help, you will learn a lot and it will help you be an overall better player, which will get you better gigs during and after high school.
3) Have fun while you're there. When I was in high school (25 years ago), I got into jazz band after only playing bass for a year and I was terrified and anxious that everyone was going to see how terrible I was. Once I relaxed a little and realized it was more fun than work to learn the tunes and a blast to hold down an entire ensemble, it became a defining moment in my early musical life. I hope you will find this experience does the same for you.
Best of luck!