The class starts and the log-ins fail.
A room of learners, an instructor at the front, and a sign-in screen that refuses everybody. The lesson turns into twenty minutes of troubleshooting in front of paying students.
What we do
We keep classroom machines on one standard setup and test the sign-in the week before term rather than on the first morning. When something does fail during a class, a person answers and works it while the instructor carries on with the room.
