My son played saxophone in jazz band in middle school and forgot his mouthpiece at a jazz festival. Trouble was, he was really good. The director made him borrow another kid's mouthpiece to play.
Aside from how incredibly gross that was, I often wonder how it made the other kid feel.
"You forgot your mouthpiece? Well, we can't have you not play. Borrow a mouthpiece from that kid. He sucks anyway."
In middle school I played the (French) horn, and one time I lost my mouthpiece right before a concert. I spent the entire thing just pretending to play my mouthpieceless instrument.
I think my teacher knew not to do a song with a horn solo; none of us actually bothered to learn the notes to the songs. The concert probably sounded marginally better without me.
A kid forgot his instrument but showed up to a performance in middle school and the teacher told him to "sit in his seat and make the gestures. No one will see you anyway."
I was too busy with my own music to watch but I know he did it. I bet it was amazing.