It brings to mind the Christmas concerts my old secondary school choirs did (and do). (More than one choir - there's a junior choir, senior choir, junior and senior special choirs, chapel choir, etc. All in one school, with some students in more than one choir.) Now, I'm talking a staged performance in the local theatre venue, with maybe 500 students in the choir. Lots of trooping on and off, and up and down, the stepped seating (think almost amphitheatre style seating on a stage), for different choirs doing different songs, orchestras and other instrumental groups coming on and off for their performances, Head Girl making a speech, etc.
In order to be in the concert, you had to attend the rehearsals. And I mean had to. And if you weren't at the dress rehearsal on the day of the concert, you simply couldn't be in the concert.
Some of the parents of the Year 7 (12-year-old) students, who had just moved up from primary school, couldn't understand why their child couldn't be in the concert if they couldn't be at the rehearsal that day. It had to be explained to them, that this wasn't like primary school, where you could just plonk them on stage in the school hall. This was effectively a military operation - everyone had to know what they were doing at any given time, or the whole thing would have looked a mess. With a minimum of 500 students in the choir, plus the orchestras and other musicians, plus stage crew, and everyone else who needed to be there, it was also a safety issue; nobody had time to baby-sit someone who didn't know what they were doing, where they were going, etc.
There is a huge difference between
-going to choir practice at school and learning the carols off by heart, and....
-also going to the dress rehearsal in the actual auditorium in town on the day of the concert
-knowing all the carols off by heart
-knowing where you stand in which line to come on and off stage
-which entrance you're coming on from, and off by
-which staircase to go up and down
-knowing the order in which everything happens (eg which carols are 1st, 2nd, 3rd, then the special senior choir, then the orchestra, then the head girl makes a speech, then the entire choir sings carol x, then the flute ensemble perform, then the special juniors come down to the front of the stage for carol z, then the orchestra comes back again... etc etc etc.)
-knowing when to use which prop in which carol (this is told, but not actually rehearsed in school)
-teachers assisting the choir mistress, saying whether or not x carol could be heard from the very back of the (large) auditorium, and so would need to be sung louder/quieter
- last-minute changes to routines, timings, props, actions, program scheduling, costume changes, etc etc.
-also, the fact that seating was sorted out at the rehearsals. We were all practically squished into the pews; there simply wasn't room in the actual concert for anyone who hadn't been in the dress rehearsal.
All of this isn't just the sort of thing you could pick up on the night. So however unfair it seems to the parents, if the child hasn't been to the dress rehearsal (along with other rehearsals), they can't be in the concert.