We've been trying to get the custom coursework thumbnail working and have been unsuccessful so far. We've tried both a page URL and a url directly to a thumbnail image. We're using the PHP client like so:
...
$materialLink = new Google_Service_Classroom_Link();
$materialLink->setUrl("https://www.oursite.net");
$materialLink->setThumbnailUrl("https://i.ytimg.com/vi/m9We2XsVZfc/hqdefault.jpg");
$materials = new Google_Service_Classroom_Material();
$materials->setLink($materialLink);
...
However, instead of the custom thumbnail showing, the default (a snapshot of our crawled site) always shows as the the coursework thumbnail.
Has anyone successfully used the custom thumbnail feature?