Currently, the Administrator has only two ways of logging off all users: (1) put the system into maintenance mode (2) set the Session timeout to some short value. A third way should be available through Site administration -> Users -> Accounts -> Bulk user actions: With selected users -> Log off immediately.

A similar option to log off all users is needed at Course level. Here is a use case: The Teacher has just made a role override or role assignment and wants it to take effect before the class starts some activity. Rather than risking that some Students fail to log off (which could corrupt the activity), the Teacher should be able to give a warning and then force Students to be logged off.