My experience:

"Two boys in my class almost went at it in between periods. The boy that started it all just kept talking smack about the other kid. In other words, he was a jerk and was a trouble maker generally. Before any physical fights started, I told the other kid to get to his next class and the other one to stay in my class for a moment. It was a luckily prevented chaotic moment". 

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Other teacher:

"In my low-income urban public school we had an actual riot, with students punching police officers, tripping the fire alarm, damaging property, and screaming obscenities. School was canceled for two days as teachers reviewed security footage and suspended kids. Of course, it was a doozy to break up. A lot of dissatisfied kids in the cities."