Recently I actually had to stop a class to address student behavior--a very rare occasion.
Having overheard--quite coincidentally--the teachers discussing the issue during the recess break before I taught the class, I was thankfully more aware of the classroom dynamics than I might otherwise have been and also was somewhat mentally prepared to deal with the situation that arose.
A student was consistently louder than the other students and they (the other students) found it very irritating. Their feelings were quite obvious as most of them were holding their hands over their ears when that student spoke too loudly for their liking (which was most of the time, apparently).
Usually I'm fairly loud and energetic when doing the "teaching words" portion of a class (i.e. I say words/phrases/sentences and the students repeat them), but, being conscious of the class atmosphere, this time when I did my "three times fast" repetition (e.g. "Apple. Apple. Apple, apple, apple!"), I purposely said it in a whisper. I was trying to gently hint to that student to lower his volume and also trying to defuse some of the tension with the other students.
Unfortunately the student only lowered his voice for the three times fast portion and went back to being noticeably louder than everyone for everything else. Some of the other students, frustrated/irritated, started saying "urusai!" (meaning "you're loud/noisy!" with a "shut up!" connotation) to the student. I let it pass once or twice, but then it wasn't just one or two students saying it once to let off steam, it was multiple students saying it repeatedly.
So I stopped the lesson and said something along the lines of (all in Japanese):
You know, if you always hear the people around you saying "urusai, urusai," you may stop wanting to speak. So please think about how your words make people feel and maybe ask in a nicer way, like "Could you please speak a little more quietly."
Also, I think it's great to be excited about English and to speak in a genki way, but it's also important to pay attention to the feelings of the people around you. Please look around and maybe contain your excitement a little if you see that it's bothering the people around you.
After that the atmosphere got a little better. The student did speak a little less loudly--sometimes. And the other students refrained from angrily saying "urusai"--for the most part.
I'm sure it will take quite a bit more time for the class dynamics to settle down, but judging from the looks on the students' faces while I was speaking, I think that they were at least really listening to what I was saying. And I can only hope that they were able to understand what I was trying to tell them.
My intention wasn't to scold them. I wanted to encourage them to think more about each other's feelings and to show more consideration and kindness to each other.
Unfortunately I didn't get a chance to talk with the teacher who was in the classroom at the time that I was giving the talk. I would've liked to have heard her opinion as to whether or not what I said was appropriate/beneficial in that situation.
(I also regret that I probably wasn't able to express my feelings as clearly as I wanted to--in general I'm not good at expressing myself when put on the spot, and my Japanese also still isn't up to par... )
I'll be at the school again this week, so we'll see how the atmosphere in the class is... I really hope I won't have to give a similar talk again...