After Haileybury, Percy went to Trinity College, Cambridge. On 13 June 1888 he was awarded 2nd Class honours in the Historical Tripos.
In the 1891 Census, Percy is enumerated at 107 Ullet Road, Toxteth Park, Liverpool; he describes himself as Teacher of Classics. At his marriage in 1895, Percy is a master at Mr CC Cotterill's Greenbank School, Sefton Park, conveniently close to his 1891 address. At Greenbank he will have taught members of the Kendall family, who moved to Hoylake and were associated with The Leas for many years. The Liverpool Daily Post of 3 Feb 1941 explains that Percy was on the staff at Greenbank from 1889, and that he taught there right up until he founded The Leas in 1898.
Percy was invited by his former headmaster Charles Clement Cotterill (who had become Secretary of the Association of Headmasters of Preparatory Schools) to submit a paper to form part of a Government Report "Preparatory Schools for Boys: their place in English Secondary Education", 1900 Cd 418. The Report gives interesting contemporary views of the functions and purposes of prep schools.
Percy, jointly with Jackie Barr, was headmaster of The Leas from its foundation in 1898 to his retirement in 1925. The main buildings are reputed to have been designed by the architect William Edward Willink (who was in partnership with Philip Coldwell Thicknesse from 1888 to Thicknesse's death in 1920 - the partnership was then renamed Willink & Dod, and kept that name after Willink's death in 1924). Willink was related by marriage to the famous Thomas Arnold of Rugby School.
In 1895 Percy had married Margaretta Lawrence, daughter of Edward Lawrence, an India-trade merchant based in Liverpool. Percy and Margaretta had just one child, Lawrence Percival 'Tim' Dealtry.
Percy must have been quite a sportsman: he played cricket for Liverpool A team; and he is referred to as "an old hand with a rod" [Millom Gazette, 25 Mar 1927].