Open up the Chrome browser and type the following URL in the navigation box:

 chrome://flags

In the “chrome://flags” page search for “Enable screen capture support in getUserMedia”.    If it is not enabled, click the “enable” link.  (This is required for viewing thumbnails of the student. )

If you've made any changes on this page, you will then need to restart Chrome.  There will be a “Restart Now” button at the bottom left of the page.   Click that button to restart Chrome.