I wonder whether an organization such as the World Harp Congress or American Harp Society would be interested in taking the music off your hands, for distribution to schools or students who cannot afford to buy it. Or, you could put your collection on Amazon or Ebay and see what happens.
I would suggest that both the American Harp Society and the World Harp Congress, at their conferences, have a booth where people with music they don’t want can donate it to those organizations and that they in turn make it available to anyone who wants it, either free or pay by the pound, the money going to the organization. This could also be done at the regional or chapter level.
I can understand why the local library wouldn’t necessarily put your entire collection in the stacks. They have limited space and most small, local public libraries have to balance out the convenience of having everything with those space limitations. They tend to keep what is most likely going to be requested, plus a few odd things, and rely on interlibrary loan for the requests for most of the unusual items.
So to make your music collection available to students in it’s entirety you will want to give it to a library that has reason to think that there will be some kind of a demand for it, and the best bet would be a library at a college that has a harp program.