I traveled with high school groups annually for the past 8 years and have often had one or two things come up right before tour where a student couldn't go along for some reason.  We took our tours during academic time - not during Spring Break - so the kids were actually missing 4-5 days of their other classes. 

 

There is no way that you can make up a performance, a clinic, etc., so I would find ways to make their time useful for the choir.  If I had music programmed for the Spring concert that was not in the tour repertoire, I would ask the students staying behind to research everything they could about the pieces that we were going to learn when we got back from the tour.  When the group returned from tour, they were able to hear about the composer, background of the piece, about where the song fit in the time period, and what the student thought was significant about that song and what the class would get by learning it.  They were never allowed to put any negative ideas into their presentation, only the positive things that the choir members would take away from singing that song.  I was then able to use their research and combine it with my own to create program notes or write spoken introductions for each piece on the concert.

 

The school never allowed me to force all students in that concert choir to participate because of the cost involved, but I always had tour activities and experiences which were so meaningful to the students involved, that after a couple of years of students not going for whatever reason, the tides shifted and they would choose the choir tour over their other activities - they didn't want to be left out of those experiences.  Because the students were not required to go, I could not give them a grade for tour participation, but the assignment for those left behind was something that could keep the students busy, benefit the choir in the long run, and keep a substitute teacher from worrying too much about having to actually teach something musical...