For a teacher's device:

Step 1: Launch the Settings app from the apps tray.

Step 2: Click on Display.

Step 3: Click on brightness and adjust the brightness level.

Alternatively, you can swipe down for the quick settings and adjust the brightness using the slider


For a student’s device: 

Student’s devices are locked and have restricted access to applications and settings. 

Step 1: Launch the Quick Settings app from the apps tray.

Step 2: Use the slider to adjust the brightness as per your preference