Update and Comment permission settings can be set overall in Groups Manager for all sub-group spaces under a top level node. Site Admins can open up a top-level group (such as Academic Classes, Locations, Athletics, etc.) and set the Activity Stream Permissions for all sub-groups within that folder. They also have the option, however, of selecting the "Allow admins to override Activity Stream Permissions" checkbox on the "Settings" tab of the top-level node.
Doing so will allow Group Space Admins (e.g., teachers in the case of Academic Group Spaces) to customize the Activity Stream Permissions for their classes. Teachers can do this by logging in, clicking on their name in the Account Bar, and selecting "My Groups" from the flyout menu; they can also access Group Manager by clicking the "gear" icon on LMS pages. They will see a Group Manager window that lists only their own classes, and they can open up the Settings Menu there and see the Activity Stream Permissions dropdown menus.