Sometimes the projector in a classroom might be on but it is not projecting the way you would like to.

To quickly set this you can press the windows key + P

A pane will appear on the right-hand side on Windows 10. (Shown below.)

The options will appear at the bottom of the screen on Windows 7.  (Shown below.)

PC Screen only/Computer only
Will not display anything on the projector.
Duplicates what is on your screen, to the projector.
Uses the projector as a second monitor.
Second screen/Projector only
Uses the projector as the main display for your PC.