fox, I've done similar things when there was no use continuing. Not often, because I'm a stubborn SOB and hate to give up, but I have done so. In one class, I just stopped in the middle of my explanation, took out my mobile phone and called a friend.
"I thought you had classes all morning?"
"I do, but this class doesn't want to learn anything. They're just wasting my time, so I'm going to send them off to lunch early. Do you want to meet me for lunch somewhere?"
I talked to my friend for about 3 or 4 minutes before hanging up.The kids were so busy waiting for me to say, "Xia ke le", they didn't notice the bell ring.
A friend of mine did have a chair thrown at him by a student. The kid was so flustered he missed, and my friend dared the kid to try it again. The second time the kid hit his target, and my friend told him the truth these kids all need to hear:
"I'm going to report this, and we're both going to get what we want. You're out of my class. You're out of my school. You're out of school entirely. You won't just be suspended for a week; because you did it twice, you'll be suspended for good. Me, I'll take a medical leave and be on paid holiday. Thank you."