This better mean I can include instruments not on the list, 'cause for you to forget the ever-so-popular saxophone on there... I play trombone, so I would've stabbed you with the slide repeatedly if you didn't have that on there :>
Front Row
Flutes- Blisseys. Flutes have that serene sort of feel to them, so what better Pokemon to choose than Blissey?
Second Row
Clarinets- I can see Audinos on this one. They do have that serenity like Blissey, but it seems to be a bit more of a haunting serenity, something clarinets can pull off rather well.
Trumpets- Nidorinas. They seem like they can stand on their hind legs and when I thought of trumpets I thought of announcing a king/queen. Well, guess what Nidorina/o evolves into? :>
Saxophones- Scraftys on this one. While I cannot see hip-hop/punk with saxes, I can see this particular Pokemon put on some shades and play some jazzy tunes.
Baritone- Loudred. It wanted to play flute but couldn't :<
Trombones- Bulbasaurs. You may ask "how", but it's simple: They use Vine Whip to hold up the instrument and extend the slide outwards :>
Third Row
Bass Clarinets- Seismitoad's definitely in the lower register, and there's already a Tympole in the percussion section.
Tenor Sax- Krookodile. The Scraftys didn't want one playing sax with them, and because Scrafty has a type advantage to Krookodile, it settled with tenor sax.
Bass Guitar- Kricketune would not be allowed to play violin/viola/cello in a band, so it had to settle with this.
Tuba- Since Exploud evolves from Loudred and I somewhat see a tuba as an even lower baritone, then it fits rather well here.
Percussion
In most the school bands I was in, the percussionists alternated parts. Since I am basing this band after my current school band, the names of the Pokemon will go below rather than the instruments.
Tympole- Its name speaks for itself. Good at timpani, symbols (hard as it is to believe), and snare.
Ludicolo- You can see it shaking maracas, right? Well, that's what it would do, along with any other shakeable percussion.
Machamp- With its four arms, it can do wondrous things. Handles timpani, snare, drum kit, bass drum...OK basically it handles any and all percussion.
Medicham- They needed another humanshape Pokemon to help Machamp out. Good with most percussion but refuses to play timpani for unknown reasons.
And that's my actual Pokemon band in a nutshell :>