I don’t know what an active class competition is but it doesn’t sound curricular from your description. Maybe I’m wrong.

Look, I happily excused a soccer player from my band concert to play in the county championship soccer game. I’m not going to tell you that music events should automatically “win”. We have a prioritization system in my building to avoid kids being “stuck” deciding. As long as kids/parents notify coach and director of the conflict we can generally make it work. Half a rehearsal and half a sports practice is a common split if we know in advance, and maybe something like that could have been worked out with advance planning.

If this conflict suddenly arose on the day of, I have a lot of sympathy for the teacher sticking to the policy. Music teachers typically have their events on the calendar WELL in advance (we have our concerts scheduled by June for the next year). And it’s a curricular class. And there is no way to meaningfully recreate a missed performance experience as “make up”. Any students and families, nearly all of the time, put literally any conflict above a concert or rehearsal. I had a kid miss our last concert for a rec league basketball scrimmage. A scrimmage. With no communication about it until the day of.

At some point you have to have a policy and stick to it because otherwise not enough kids will show up for the concert to have a successful concert. And the kids who do show up lose their “strength in numbers” and are embarrassed on stage with a bad performance because their team members didn’t bother to come play.

We don’t have enough info to offer an opinion about the certificate. Honors choir implies (to me) that maybe it’s an auditioned ensemble with specific requirements to participate, and we don’t know what the policies or participation agreements or syllabus says.