We have reports that several libraries have successfully registered for Office 365 Education plans at http://products.office.com/en-us/academic/compare-office-365-education-….
The initial form you fill out is pretty simple, and looks like the screenshot we've saved at http://oplin.org/files/officetrial.png.
Note that it is set up as a free trial, and Microsoft "reserves the right to verify eligibility at any time," but so far they seem to be allowing the educational discount for public libraries.
After you click on the "Create my account" button, you'll eventually be asked to make some DNS changes to verify your domain. If you have trouble with these questions or any subsequent questions, you can contact http://support.oplin.org and we'll help you out.