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.|
|Duplicate||Duplicates what is on your screen, to the projector.|
|Extend||Uses the projector as a second monitor.|
|Second screen/Projector only||Uses the projector as the main display for your PC.|