Once a course term ends in Schoology, the course goes into "Archived" status in Schoology. While you will still be able to access your "archived" courses (and copy content from there), the best bet is to have all course information and materials saved in the Personal Resources area for later use.
In any course, a teacher can use the "Options" pull-down to get to the "Save Course to Resources" option. If a teacher wishes to save the course content to a group for sharing with others, s/he can just select the Group as the destination. After choosing a destination, a folder will be created in that location with all of the course content inside.
As we continue into the next school year, you may want to consider building content in the Resources area instead of a course as it simplifies this issue and makes copying to other courses or sharing with others much easier. Plus, if you want to work on materials over the summer, you can build inside of Resources and then copy to your courses when they are available for the fall.
For those leaving the district, you can always create a free "Instructor" account with http://www.schoology.com using a personal email address and then "connect" with that account. If you create a "Collection" in your Personal Resources area, you can then share that collection with your personal account. See Schoology's help area for more information on collections.