School Admins can find those accounts by filtering accounts by delivery errors.
Go to the account index page (where all accounts are listed).
Use the status filter and select Invited - Error.
This filter shows accounts where the invitation email could not be delivered. The system marks these when the inbox is full, the email address is invalid (for example a typo), or another delivery issue occurred.
From that list, you can review the accounts and correct the email address if needed before sending another invitation.