The first thing you will need to do is add a page in the appropriate area of your site. This would most likely be under the Faculty and Staff Portal page, but this is not a requirement. This would become a Class Directory page.
You will set up this new page with the following attributes:
Page Type: Directory
Page Format: Group Directory
Once you save the page you can then select "Academic Classes only" from the Group Directory Properties.
Go to your Advanced Controls and select the faculty role as authorized. (If you created this page under a Portal page, this is not necessary - the page will already inherit the Portal page's visibility restrictions.)
The other step you need to take for this to work is in a different place. You will need to go to:
Site Manager>Site Administrator>Site Modules>Groups
You will see a section called "Guest Access" You will need to check "Faculty and Staff."
Notice that you can select other roles and grant guest access to view academic group spaces, so when you are ready you can do the same thing for parents, too. Also note that you can specify who can see a Roster.
Now when a faculty member logs in they will see a page called Class Directory. They can go there and search for any academic class and view it as a guest.