Must the "Reading Medal" be returned to the school if books were borrowed but not read, or only a small part of them was read, due to lack of time?

If the school awarded the medal based on the assumption of extensive reading, and it did not know the truth of the matter, then it is not permissible to take it, due due to the Prophet’s (peace and blessings be upon him) saying: "The property of a Muslim is not lawful except with his willing consent." The prize must be returned to the school. If they give it to you after being informed, then it is permissible to take it.