The issue was resolved on March 22 with a Google Classroom app update.
ISSUE: When students go to use the PDF editor on an assignment (by pressing the pencil in upper right hand corner), they are taken to a black screen where they can write, but are not able to see what they are writing over.
DETAILS: This issue started after an update on Thursday, March 4. This is not related to the web filter or relay in any way. Both have been tested and ruled out. Older versions of the app are still working. Google is aware of the problem with their update and hopefully will have a fix soon.
https://support.google.com/edu/classroom/thread/100756781?hl=en