A common issue that can happen on a MacOS (Apple) classroom computer is that a new user can not log into the system, as they are presented with a different user's login screen asking for a password.
This happens when the previous user has forgotten to log off the system, and the security features in MacOS require a password to continue after several minutes of inactivity.
The only course of action you have in this situation is to force power-off the Apple computer and then restart the system.
Step 1 - In the AV rack, locate the power button associated with the Apple computer. They are typically labeled "Mac Power"
Step 2 - Hold the Power button down for 8 - 10 seconds (in certain rooms that have a USB switcher in the AV rack, the light indicator for the Apple computer will turn off on the switcher, letting you know the computer is off and no longer sending a signal).
Step 3 - Release the Power button, then press the Power button again for about 2 seconds. This will turn the computer back on and allow a different user to log in (as previously noted, in certain rooms that have a USB switcher in the AV rack, the light indicator for the Apple computer on the switcher will come back on, letting you know the computer is on and sending a signal).