Site administrators can set a default layout for all of the subgroups in a node in Groups Manager. The default layout will only apply if the group spaces has not been customized in any way.
Site Admins can update every Academic Class in Group Manager. This is done in Group Manager > Academic Classes > Group Space.
Open Group Manager, and click on the "Academic Classes" node.
Click the "Group Space" tab.
Configure the home page layout that you want to apply to all of the Academic Classes (note that you will NOT be able to edit content elements here, you can only drop empty elements into the layout. Editing the actual content of content elements must be done in the LMS itself.)
Click "Update Settings" to propagate the layout to all the groups in the "Academic Classes" node.
Please note, this will apply to all group spaces below that node. Also, there is not a way to set multiple default layouts per top level node.