The All Computers list contains hosts that you've added to your list and have access to control via ARD. The Scanner lists all of the hosts found during a network scan. This could include hosts that you don't have access to and machines that are not even Macs.
3.
A teacher can observe multiple computers, a teacher can share his screen with the entire class, or a teacher can share a student's screen with the rest of the class.
4.
Select the target computers, and send it the UNIX command: diskutil repairPermissions /.
5.
System Overview, File searches, Storage information, Memory information, Software Differences, Software Versions, Application Usage, User History.