An update to this: now there's a new feature in the teacher interface to make it super-easy to make sure your students are seeing what you want them to.
Open any student group page (listing the students), and look for the link "sign in as" for any student. That's immediately take you to the student's interface as that student, so you can check their list of assignments, etc..
You're still ALSO signed in as yourself, so in the orange bar you'll see links to go to the student's home page, or the teacher's home page, and you can click a sign-out link for just the student account to close that session.
It's a little more complicated to explain than to just use it -- try it out and you'll see!
-Rob