This morning, more joy with Lab B teacher station, which had been running and logged in all night. Attempting to log off proved fruitless -- the system presented the log off dialog box, but just did nothing when I clicked on the button, apart from dismissing the dialog box -- and it was impossible to shut it down either. My guess is that many users are simply giving up on this and leaving it logged on. I had to shut it down with the power button and restart it.