The Adventure of the Three Students

"The Adventure of the Three Students", one of the 56 Sherlock Holmes short stories written by Sir Arthur Conan Doyle, is one of 13 stories in the cycle collected as The Return of Sherlock Holmes.
Sherlock Holmes and Dr. Watson find themselves in a university town when a tutor and lecturer of St Luke's College, Mr. Hilton Soames, brings him an interesting problem. Soames had been reviewing the galley proofs of an exam he was going to give when he left his office for an hour. When he returned, he found that his servant, Bannister, had entered the room but accidentally left his key in the lock when he left, and someone had disturbed the exam papers on his desk and left traces that show it had been partially copied. Bannister is devastated and collapses on a chair, but swears that he did not touch the papers. Soames found other clues in his office: pencil shavings, a broken pencil lead, a fresh cut in his desk surface, and a small blob of black clay speckled with sawdust.