Hi @Dan613,

My first guess is that the students are logged into a personal google account, or multiple google accounts at the same time.  The way google handle multiple logins has been a thorn in my side for years.  For whatever reason, google seems to just check the "active" account for access instead of checking whether any of the other logged in accounts for the user can access whatever the link is.  When we like to google thinks shared with specific individuals or our institution only, we generally direct users to open the links in an incognito window and log in with their school credentials.  You could also change the sharing to "anyone with link" but that could cause unwanted access. I really wish Google would do something to fix this, but I think it would be easier to change the earth's orbit than it would be to talk to someone from Google.

Hope the info here might help your situation!

-Chris