Teacher's MacBook's that have the Big Sur OS have had trouble recognizing external DVD players.

• Here is a step by step guide to implementing this fix.
• Plug in the external DVD player and insert the DVD.
• Restart the MacBook with the external DVD player plugged in
• First you go to Disk Utility and run "First Aid" on the disk drive.
• Once that is completed download VLC player, be sure to click and drag the VLC icon onto the applications folder when the pop up box appears. 
• Once it is fully installed, do to system preferences, click the CD & DVD settings, check the box that puts a eject button at the top of the screen (This is a master eject button that will eject any disk no matter what.)
• Once that is completed click the "When you insert a DVD" pull down button and click "Open other application" and search for VLC in the pop up that appears. That will make VLC the default player.
• At this time the DVD symbol should appear on the desktop. Once it does open VLC player, click open disk and there should be a description of the DVD in the player, click that and the movie should start playing.

This is far from a perfect or long term solution but it should work for now