I just ordered the book Lang mentions in this article.
In my experience — having observed many dozens of college courses over the past two decades — most faculty members eye the final minutes of class as an opportunity to cram in eight more points before students exit, or to say three more things that just occurred to us about the day’s material, or to call out as many reminders as possible about upcoming deadlines, next week’s exam, or tomorrow’s homework. At the same time, we complain when students start to pack their bags before class ends. But why should we be surprised by that reaction when our class slides messily to a conclusion? –James M. Lang, The Chronicle of Higher Education