If you need to leave your PC unattended (out of visual sight) then in the interests of data protection you are required to lock the screen. On a Windows PC, this is simple to do via the key combination shown below (Hold down Windows Key and tap the “L” key:
Your screen will then lock and can only be opened again with your password. Only set this function if you intend to return to the classroom as this can stop others from logging in and using the PC.
If you need to keep a slide viewable on the projector, use the freeze function of the projector. This can be turned on and off