He was ordained deacon by Richard Terrick, Bishop of London, on Christmas Eve 1769, and for a short time he served curacies at the nearby parishes of Willesden and Kingsbury. Parr obtained a licence from the bishop, on 14 October 1771, and he opened a school at Stanmore. Perhaps hastened from need of a female superintendent at the school, he married Jane Marsingale the next month. Despite his taking between 20 and 40 pupils from Harrow with him, the school failed after five years, mainly because it could not compete with the extensive interests of Harrow. During his time there, however, he successfully introduced various curriculum developments, particularly the teaching of English and its grammar (as opposed to purely Latin), on which topic he published his "Introduction to English Grammar", which appeared in 1765.