Tbf, did you confirm their lack of library cards, kindles, and books on tape either MP3 or actual physical cassette?
Could be they just had one each checked out in the nightstand or in their work bag, or had returned the last, but hadn’t checked out another yet because X, or had a huge library of digital files either bought or “acquired” from a certain archive, or something like that. I assume a school admin has access to at least a library of sorts, whether they want to read All Quiet on the Western Front or To Kill A Mockingbird again is another question.
Never know! Unless you had a warrant to search for books, then you probably would know.