There are several common fixes for sound issues on a Macintosh in a classroom setting:

HDMI or Mini DisplayPort connections

If connected via HDMI or MiniDisplayPort cable, be sure that the proper sound output is selected:

1. Go to System Preferences >> Sound >> Output tab.
2. Under the Name column, locate "Crestron" and click on it.
3. Disconnect and reconnect your HDMI or Mini DisplayPort cable. Sound should pass through speakers.