You are lucky to have got away with 6 Nooks, usually 6 devices in total (including the computer).
It may be that if you use different OS level users on the laptop you will be able to register them with different Adobe IDs.
Also, you can keep on deregistering one ID (ctrl-shift-D, cmd-shift-D on Mac) and reregistering to the other
... not very convenient, and I'm not sure the Adobe registration server might not lose count and think you've got more registrations than you do.
If you do arrange two different Adobe IDs, you will find you cannot share books between them.
Adobe may have special arrangements for schools and libraries, or possibly ask Overdrive themselves.
I suspect Overdrive might be more helpful as this is their core business,
where Abobe seem to treat the whole ADEPT DRM thing (software and infrastructure) with a total lack of professionalism.