Reflector Teacher allows you to easily emphasize one device when multiple devices are connected.

Emphasizing a device makes it larger than other connected devices to spotlight a certain screen.

To emphasize a device, open the Reflector Teacher menu and then tap the gray emphasize button. 

The button will turn green, and the device will get larger, placing emphasis on the screen. Tapping an emphasize button for another device will emphasize that screen and remove emphasis from the previous.