Well, we do, but the bell is programmed to make the sound of several dozen people shoving their notebooks into their backpacks.
One of the very few occasions in which I'll become cartoonishly aggrieved with students is when I hear that sound, check my phone, and it turns out I've got five minutes left.
Me in that situation: "There's a very good chance that I'm going to say something important in the last few minutes of class, so if you put your book away early, you're going to both slow me down by confusing me and deprive yourself of info you're going to want to have."
"I try never to keep you past the end of class, and if I occasionally do, I apologize. It will never be more than a few minutes, and it'll definitely be more than balanced by days when I let you go a few minutes early."
"If you need to leave exactly on time or a little early, you can always slip out quietly. But in the absence of that, please don't start packing up before I'm done. It'll just screw us all up and keep us here longer."
That speech usually works, and if it doesn't, a brief recap the next time it happens pretty much always does.
