Genuinely shocked at those saying YABU.
In school my friends and I would sing frequently, we were in a band together and were asked to perform at assemblies frequently - Literally no one had an issue with it as we were all still working, it wasn't remotely disruptive.
Many other girls would sing during class too. (In groups in turns, not all at once with different songs, that would be a racket!)
But then it was an all girls CofE school.
Idk if that makes a difference.
We were asked to stop I think once in the entire 4-5 years we were at that school, and even then that was only because it was time for the teacher to talk again, not because it was annoying anyone.
Idk what schools are like these days but it was pretty normal when I was younger for there to be about 20 minutes of teacher led stuff to begin with, then we would be given work to get on with via textbooks and working as groups for the majority, then a wrapping up portion at the end for about 10 minutes.
As long as everyone was doing what they were supposed to, there was no issue.
I did get in trouble for doodling whilst the teacher was talking one time, but that was a quiet activity that helped me concentrate/listen to the teacher better and wasn't disruptive to others, so after explaining that I found it easier to concentrate whilst doodling, it was no longer an issue :S