Why did Taplow come to school ?

Taplow had come to school for extra work under the orders of Mr Crocker-Harris. As he tells Mr Frank, he would have preferred to play golf but since Mr Crocker-Harris was leaving and Taplow had already missed a day of school last week, it was imperative for him to come. Also, Mr Crocker-Harris was unlike any ordinary master, so he had called in Taplow for extra work, failing which he would not have been conferred his remove to graduate from class.