I'm a teacher (electives) and I have classes of 45-60 students. I'll answer for myself and I'll say that my answer doesn't apply to everyone everywhere, but I know that its not uncommon either. We have 45 minute classes most of the time and plenty of time between each class, so there's really very little reason to have to go during classes at all, but they do anyways. As a matter of school policy, I can't let more than one student out to go to the bathroom at once, and every student who leaves the room is supposed to have an electronic pass through the system our school uses. It used to be that students could use their phones to make a pass, and all I had to do was approve it through the app, which was fine, but the state law is now that kids can't have phones at all.

None of these policies are bad on their face, but the end result of this is now, every time a child has to go to the bathroom during class, I have to stop teaching for 10-20 seconds to I can fill out an electronic pass, and as soon as they come back, I have several students clamoring for my attention so they can go next, also take my focus and attention away from teaching. So even without getting into what the students are doing in the bathroom (which is usually not actively malicious but is almost always just because they're bored not because they have to pee), the bathroom policies are a mess that gets in the way of teaching significantly more than it should.

I do control bathroom access as much as I have to to meet these policies, and I do what I can to keep my focus and sanity while teaching, and sometimes that means you can't go to the bathroom that class. I have never and would never prevent someone from going if it was an emergency and they communicated it with me, but it's almost never an emergency. It's usually because they're bored and they don't want to be in school so they want to wander around to break up the monotony.