Administrators of an ArcGIS Online Organization account have important responsibilities. They control all permissions and settings in the Org, including invitations, entry, and privileges. So I am astonished when teachers seek assistance because the ONLY admin in the Org has left the school. I'm gobsmacked when I learn it happened months ago. Good Org management means there should always be someone who can get into the guts of the Org and do key administrative tasks within 24 hours. This is so easy to set up in advance, and so much harder after someone is gone.
Let's say Alex launches the Org. She invites 10 teachers into the Org, gives them Publisher status, helps them log in, and sets up single sign-on so all 7th graders can be Users in the Org. This is a good start, but not enough. Alex needs to find at least one helper, someone who can do tasks when she is out of contact. So Alex promotes Billie and Carly to admin. Done, right? Again, it's a good next step, but not enough, because Alex is still the only "primary admin."
Primary admins receive all important communications from Esri about ArcGIS Online in general or about this one specific Org. They receive emails when a user runs out of credits or requests a password reset. There is ALWAYS AT LEAST ONE admin set as primary. Any admin, and any number of admins, can be given primary status, or have it revoked, unless that admin is the only primary, and every Org should have at least two. To configure primary admins (shown below as "Designated Administrators"), follow the 1-2-3-4-5 click sequence in the image here.
Esri is happy to provide an ArcGIS School Bundle free to schools and clubs for instructional use. It's a powerful instructional resource. But as fans of a certain "superhero web guy" know, "With great power comes ... great responsibility." Good ArcGIS Online Org administration is a process, not an event. See more essential considerations for good Org administration for schools at http://esriurl.com/agoorgsforschools.