These steps require the computer and the External Display (i.e. TV, external monitor or AV System) to be connected and both device be active.
Step 1 - In the top left corner of your screen, click the Apple
Step 2 - Select System Preferences
Step 3 - Select Displays
You will now be shown two windows, one window will allow you to adjust the visual settings on the laptop's built-in screen and the other will (by default) allow you to adjust the external display. Each window will have a drop-down list of connected displays. Unless you have changed the computer settings, the computer's screen will be identified as Built-in Display. The external display will be represented by either Display or the name of the device. A common name to be displayed if connected to a classroom AV system will be ExtronScalerD. ExtronScalerD is how all Classroom AV switches present themselves when connected to a computer.
Step 4 - In the window labeled Built-in Display, select the center tab labeled Arrangement
Step 5 - On the left side of the window towards the bottom (above the drop-down for AirPlay Display, you will find a checkbox labeled Mirror Displays.
When Mirror Displays is checked, the same content will be sent to the Mac's built-in display and the external display. If the box is unchecked, the two displays will show different content.
If you are making this change on a teaching station computer in a teaching environment, please be sure to either reboot the computer at the end of class or make sure that the Mirror Display option is Checked. This will ensure that the settings are in the default condition for the next person teaching in that space.