Advices please for my 8 year old son ?

My son in year 3 has been barred from borrowing from shool library cos one of the school teacher lost his book. End of Term 1 , just after he went out to the playground with newly borrowed book from library, he asked this teach on duty if she would like to keep an eye for his book and she agreed and said to him just to leave the book on the floor near where she was standing . But when he came back , the book was lost. And the teach on duty was very indifferent and simply said she was busy supervising the playground. He was afraid to confront the teacher cos he knew she sometimes wasn't nice. I honestly didn't think that was his fault unless he was blamed for trusting his teacher. It wasn't about money /fine and I don't wish to get involved so early cos I wish him to learn more about trust, keeping promises, fairness and courage. I do understand it could be very difficult for him to confront some BIG authority like her ? Any advice pls ? Appreicate.