I need a little bit of help.

During Jazz band, my teacher gave me the option to compose a piece and it would be played by the school band at next years concert. The only problem is that for me, writing for a full band is a little overwhelming. Any tips on composition?

if you can i would recommend getting a sheet music midi program

I already have Finale

Analyse each instrument, each one holds a different feel and works brilliantly at a given time. If you want, I can send you some stuff to analyse and break down so you can start working on it? Thankfully it'll be for next year but this should be a long and slow process if you want it to be really good. I'm used to composing for orchestras and Jazz bands so hopefully I can help you out.

ok well something i do is make a melody on with my sheet music program, or my guitar. then listen to it and hum a bass line or something. but idk how many instruments are in a jazz band

That would be great if you could do that Jazz

I would start listening to other compositions that utilize the instruments you are going to be writing parts for. This way you can hear how some instruments really shine and should be used. Break down the pieces and see what kind of scales and runs they do. You need to balance it out between something that is PLAYABLE and sounds GREAT

Not a very easy task

agree with Jeff to do compositions with parts. Its good to start from rhythm section, so first think whole song as and say simple beat like bassdrum, hihat and snare, then arrange them from parts I assume you know basic stuff, do couple bridges and then you left three options.

Do Bassline, melody or chord progression first. then add the others. As I know your skill, you know scales pretty good and how the things work I believe its not a overhelming after you started working.

Listening greatest of all instruments helps a lot. after you have whole song as an raw version that is made for simple content, you can tweak and add things there, but again Jeff mentioned its very important to make it playable, because there no use for anyone if your song is too hard to band to play, assuming they are not proffessional players :).

Good luck to your work m8.
Empirism

I forgot to mention that this is for 13 and 14 year olds :P.

In that case you're going to need to take serious account of the individual capabilities of the performers you're writing for.

apologies. Empirism's already made that point.