Google Docs, Slides and Drawings rolled out a convenient feature in June 2019 to preview link details. When you type and click a link in Docs, Google will automatically display the title, thumbnail and an excerpt of the link for you. It also converts the text part of link to the site title.
But, for schools, a coin has two sides. Given that students like sharing resources, the feature exposes potential risks for students to find violent, nudity or disturbing content. As the trick part, your network firewall is unaware of these links and failed to filter them because Google carries the data as the proxy for students.
First of all, you are better off blacklist these inappropriate domains or links in your network firewall. At least the students are not allowed to open these links in new tabs and see the whole page.
Then, install Safe Doc. It's a chrome extension that blocks certain inappropriate features in G Suite to protect your students.
When Safe Doc is deployed in your PC or your students' Chromebooks, without configuration, it blocks the link preview feature in Docs, Slides and Drawings. You will see the link pop up is gone. The rest buttons are copying link, editing link and un-linking.
The blocking strategy in feature-level with Safe Doc and network filtering protects your students from finding harmful contents.
To configure Safe Doc to not block the link preview, set BlockDocLinkPreview policy to false. See more details in Safe Doc Configuration.
If you have questions, please leave comments below.