Here's a story:
My boss was in line, waiting to go through the security check at a European airport. The line was about 30 people long, and he was about four back. Two couples about 20 somethings, Western European, walked over and got in line in front of him, never looking at the line or acknowledging it even existed. Everybody behind looked at each other, but didn't say or do anything.
My boss, who is about 60, and looks rather "bookish", took the bull by the horns and said, "I believe you need to get into the back of the line, we were all here first." The two guys looked at him and smirked, and the women looked at each other and started to giggle, and then went back to talking.
Then, a person or two back, a tall athletic African American guy, about mid 20's said in a booming confrontational voice "He said you all need to go to the back of the line." and got out of line, walking up to them and pointing to it. Several other people joined in, pointing to the back of the line, telling them "back there", and one older woman saying "What's wrong with you!?" to the two women.
Well, the two guys lost their smirk, and I bet their manhoods shrunk four inches at least. The women put on an affronted look but refused to make eye contact as they made their way to the end of the line.
The tall guy said, "I figured if anyone else said anything, I was going to back them up, but I didn't know how people would react if I said something first." Another couple said, "We get so tired of seeing rude behavior like that, that when you said something, we thought it was time for us to say something too, to stick up for you sticking up for us."
Moral of the story, someone has to be the first to react. If you were in the classroom and threw a textbook while yelling "throw something!", someone else might copycat and throw his book. Then someone pitches her laptop, and the ROTC guy throws a couple of chairs, and the jocks start pitching desks and tables. With any luck, he gets his bell rung, and a hog pile ensues.
If however, you get in line against the wall, you reinforce that plan of action....