Is preventing a friend from having explanatory notes for a school subject, under the pretext of needing the teacher’s permission, considered concealing knowledge? Is it permissible to give these notes for a fee? What is the recompense for one who conceals knowledge?

First: The warning regarding the concealment of knowledge is specific to knowledge that is obligatory to disseminate. If the knowledge mentioned in the question is not obligatory to disseminate, then the student or teacher does not incur sin by concealing it.

Second: If the teacher is the one who prepared the papers, it is permissible for him to retain the publishing rights and to charge a fee for allowing their publication, as this falls under intellectual property rights. Therefore, your friend is not permitted to give you those papers except with the teacher's permission. However, if the teacher does not object to the publication of the papers, then publication is permissible without seeking his permission, and your friend is allowed to refrain from publishing or to profit from the papers as long as he owns them.

Third: If the student is the one who wrote, collected, and arranged the papers, then there is no blame on him for withholding them from you, and it is permissible for him to profit from them, whether the teacher has granted him permission or not.