This paper discusses the trial of Thomas Hopley, accused of killing his pupil Reginald Cancellor in 1860 during an act of corporal punishment. The case provoked immediate sensational interest and became an important defining point in how corporal punishment is treated in British law. Established by this trial was the test that any corporal punishment, most particularly that which went on at school, must be "moderate and reasonable", a test so central to discussion of punishment that it is described by modern legal experts as being of continuing relevance. Throughout the nineteenth and twentieth centuries the Hopley trial was an important precedent in any legal consideration of corporal punishment. (Contains 111 footnotes.)