Does buying books for students and then discovering their unsuitability for them constitute a form of betrayal of trust?

Your work in selecting books for the school library is critical and a great responsibility. You must exert effort in choosing beneficial books that are free from anything that contravenes Islamic law or encourages vice. If you purchase an unsuitable book after exerting your utmost effort, there is no blame upon you, for Allah Almighty says: "So fear Allah as much as you are able." It is preferable to return the book if possible.