Anti-Climacus

WELL: This is almost a good idea:

"I don't mind when students slip into the back of a lecture hall late, but when the doors are in the front of the room and the teacher's voice is easily overwhelmed, please have courtesy and remember that a late entry imposes costs on punctual students, and that 12(!) late entries in the course of a 50-minute class tempt me to resort to vigilante retaliation. If I'm ever an instructor, I will lock the doors to my classroom a few minutes after I start lecturing, to internalize the externalities."

I say almost a good idea, because it's pretty clear that if you did that, most people would loudly try to open the door multiple times, then finally knock on it until someone let them in. In other words, the solution would probably be worse than the problem.