+ Remove them from their shelves, and send a letter saying they do not have the books available. This is the cheaper/easier option, and several schools have already opted to do so.
+ Make a count of all the books, provide location information about the storage of the book, including its exact geographic location on their campus, and trawl their records to determine precisely how much they spent on each of the books in an itemised manner [0], and send a report detailing all of this.
[0] Schools receive books generally in either donations, or in groupings that are not itemised, making determining the cost of an individual copy of a book extremely difficult. That's before you get to poor financial record keeping that is also rife in school libraries.