Sharing google doc via link - require sign in not working

I am running into some road blocks when sharing a google doc with students.  I think it may be the way our domain is set up:

staff = staffname@xxxxx.xx.xx.xx
students = studentname@stu.xxxxx.xx.xx.xx

I'll post the link to a doc in our class moodle.  I have the box checked on the google doc to require sign in.

However even if I have the students sign in to their google account first and then go click on the link to the share document, it will take them to a page asking them to log in.  But the log in page is the teacher log in page (@xxxxx....) and not student (@stu.xxxxx..).

One way I have found is to share the doc with the students by entering in their email addressee under the share options.

Another option that was recommended to try was to create a generic student account, put the docs there and share that way.  Unfortunately in a student account, the share link option also requires @xxxxx.. and not the @stu.xxxxx

I guess that means stu is not a subdomain? That kinda defeats the purpose of being able to lock access down to just teacher and students. 
Suggestions?