Try these steps in order to resolve:
Confirm the NetClassroom link is correct: BB555212
Confirm single sign on is setup in system options in Blackbaud NetCommunity
Confirm the page security is not marked for everyone
Note: Page cannot be marked as "Everyone" - must require an authentication process in order for single sign on to work correctly.
Therefore, the user must be associated to a role in Blackbaud NetCommunity and that role should have access to the page you are attempting to access for the NetClassroom or FAWeb link.