Where can you mention someone in Google Classroom
Mentions work in a comment on an announcement of the Classroom Stream. You can not mention someone in the body of an announcement. Teachers can direct an announcement to a specific class or student while creating a post.
At the time of this writing, mentions in Classroom only work by typing their email. Typing their name does not work.
Google Docs, Sheets, and Slides
Comments in G suite files can include mentions. You can mention someone by typing their email address or name. (Classroom only allows you to mention by their email address.)
You may use comments to assign tasks or action items to people. This only works on G suite files. To do this, mention someone while creating a comment. Then click the “Assign to” check box. The assigned person will be responsible for marking the task as complete. You may reassign the task to another person by adding another comment.