Let them know that you're interested in the other trombone player and would like to sit next to him. See if they're willing to switch seats with you.
2. If your friend is not willing to switch seats, you can try to talk to the other trombone player directly..
Express your interest in him and ask if he would like to sit next to you. Be confident and direct, but also be respectful.
3. You can also try to get to know the other trombone player better outside of band..
Talk to him before or after class, or try to find some common interests that you can bond over. The more you get to know him, the easier it will be to ask him out.
4. Once you feel like you have a good connection with the other trombone player, you can finally ask him out..
Be clear about your intentions and let him know that you're interested in going on a date with him. Be prepared for rejection, but don't give up if he says no. Just try again another time.