Ye of Little Faith by Rog Phillips, Science Fiction, Adventure 12+

📕 The disappearance of John Henderson was most spectacular. It occurred while he was at the blackboard working an example in multiple integration for his ten o'clock class. The incompleted problem remained on the board for three days while the police worked on the case. It, a wrist watch and a sterling silver monogrammed belt buckle, lying on the floor near where he had stood, were all the physical evidence they had to go on.