Please refer to the following instructions for utilizing Chromecast devices:
Cast from Chrome to your TV/Projector
Cast from Chromcast-Enabled Apps to your TV/Projector
Cast from the YouTube app and YouTube.com
Chromecast-enabled site vs. casting a tab
In most cases, the Chromecast will be on HDMI2. Make sure your projector is on for about a minute before trying to locate the Chromecast in your room. It will show as BB-XXX-CC, where BB is your building code and XXX is your room number.
Hit the 3 dots on the right hand side in chrome, go down and hit cast, go down to the bottom where it says sources, and click cast screen if you would like to project something from your desktop other than tabs opened in chrome. Make sure you click on your classroom to project.
If you want to cast your entire screen, choose the appropriate option in the sources button in the cast dialog.