In the scene after the Titans win the regional game you hear the band playing in the background. If you look at the people in the scene the band members are walking around talking to their friends but you still hear the band music.
It was probably a pep rally. I know at our pep rallies not all of the band members play. Some of us take turns playing or we just sit there and goof off and talk to our friends. Same thing in the stands at games. Not everyone will be playing. Some just goof off.