I have come to a realization. Some kids will not cooperate with an adult until the adult becomes angry. I can ask kids politely to get quiet several times and get no cooperation, but as soon as I raise my voice and show them that I am angry, they will get quiet right away. Sometimes I will pretend to be angry just to get them quiet, then quickly return to a polite person and continue teaching. When I yell at them, they get quiet for a few minutes, then the cycle starts over again--like they forgot how mad I was a few minutes ago--and I start asking them to get quiet again nicely--until I get angry again--AAAGGGHHH!!!.
I asked some of my classes if that's how it is at home and--guess what--it is. Their parents can't beat them, so they yell at them. I told them how it was when I was a kid:
"See, when I was a kid, if the teacher or parent got mad, that usually resulted in a whoopin'. You don't forget about a whoopin' in just a few minutes. You usually remember that for the rest of the day--especially if getting a whoopin' at school meant that you would get another whoopin' when you got home. The pain and humiliation of getting a whoopin' would sometimes cause you to behave for even a few days."