If my child had a medical issue or the lessons were overly long, you bet your bottom dollar I'd have a problem.
As a teacher I did let them go. Better to have a kid miss two minutes and be able to concentrate for the other forty, than have them distracted by their discomfort. Obviously you deal with the p!ss-takers accordingly ('scuse the pun).
As adults we can pretty much go whenever we like, unless we're in a job where that isn't possible or we're so micro-managed that we dare not. I certainly work better when I'm not trying to distract myself from the falling rain or the sound of an opened tap. So I don't see the point in making children suffer that way.
I think I would be upset if my child had an “accident” because a teacher wouldn’t let them go, additionally the risk of bullying if they did is too great. Could always make it clear to the children at the start of the term to go first and not have to ask during lessons.
I was born a green, I live a green and will die a green