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.