Needing both funds and intellectual stimulus, he decided to return to Harrow. From February 1767 to the close of 1771 he served under Robert Carey Sumner (by that time a personal friend) as head assistant at Harrow, where he had Richard Brinsley Sheridan among his pupils and enjoyed an income of about £100 p.a., consisting of £50 salary and about the same amount in fees from private pupils. When the headmaster died in September 1771, Parr immediately applied for the position, but was not selected. Harrow demanded every applicant had a Master of Arts degree; an honorary one was swiftly granted to him from Cambridge, where all of his previous teachers spoke highly of him.