He used his own ration and money to buy extra food for the boys of the House during World War II, which made him very popular.
The first, in which he was somehow simultaneously the King of Spain and the Lord Chief Justice of England, was the trial of Christopher Columbus (played by the Head Boy) for the heinous crime of discovering America.
The boys (Charterhouse was all-male in those days) all loved this (and also his habit of throwing blackboard rubbers at the headmaster!).
Sellar's contribution is particularly noted in the comic exaggerations and name confusions; his knowledge of English literature also inspired the book's many literary allusions and pastiches.
After completing the book, Sellar worked with Yeatman again on a sequel, And Now All This, a parody of general knowledge, including subjects as diverse as geography, knitting and topology.