Students sometimes walk out of the room during my lectures.
I know that it is considered not polite. Once in the room they should suffer through the lecture until the end, advocate proponents of good manners. But why? If they are bored, if they know the material already, if they are hopelessly lost, if they are suddenly taken by a bout of sleepiness, or if, for whatever reason, they are not getting anything out of attending class, why should they stick around?
In the room where I teach, it is easy for people near the side alley to leave discreetly without, I think, distracting anyone. I see out of the corner of my eye the figure of someone leaving, but it does not interrupt the lecture in any way. Of course I hope that students are benefiting from lectures, but if not, isn't it slightly hypocritical for them to stay?
I pretend to teach, they pretend to learn, and it becomes a charade.