When you're in a conference room or class room and need to project, connect your PC using one of the cable connectors in the room, press the Windows logo key + P, and then choose one of the four options:
PC screen only. You'll see everything on your PC. (When you're connected to a wireless projector, this option changes to Disconnect.)
Duplicate. You'll see the same things on both screens.
Extend. You'll see everything across both screens, and you can drag and move items between the two.
Second screen only. You'll see everything on the connected screen. The other screen will be blank.