In 1828, the land on which the church and school were eventually built, was donated from the private holdings of the Anglican minister at that time, named Rev. William Macauley. Father William Fraser was the Catholic priest from St. Michael's parish in Belleville who negotiated for the land with Rev. William Macauley. (Speculation abounds as to the origins, therefore, of the name" St. William" for the church). There is evidence to suggest that some form of Catholic School was operating in Picton as early as 1836, probably using the church building.

According to a table dated Toronto, April 30th, 1855, the only school in the Prince Edward County was a formally registered Catholic School in operation in  Picton ( Hallowell) dating from 1854. The local residents and Prince Edward County archivists believe that the school was a wooden frame one-room school house on the site of what is now St. Gregory Catholic School. It was probably known as St. William school, since the church which existed beside it was named St. William.

The school was officially referred to , however, in the early correspondence with Rev. Egerton Ryerson, the then Superintendent of Education in Toronto , as Hallowell Separate School No. 15 . ( In the early correspondence the local superintendent was looking for government money ,even then, to run the school.)

In 1880, the wooden frame school was dismantled and on the site was erected a brick one-room building which was re-named St. John's Catholic School. When the new larger building was erected, the cemetery had to be moved to its new site, Mt. Olivet, giving rise to many stories about hauntings, none of which has been verified.

Mrs. Goodwin was the teacher in the school from 1906 -1937. In 1943, the school was staffed by the Sisters of Providence.

In 1955 the first addition was built on to the old St. John's school, but until that time it remained a one-room schoolhouse. The current school library occupies that old "schoolhouse" part of the building and the original entrance still exists. The new addition consisted of three classrooms. There were 79 students in attendance. Teachers at the school were Sister Catherine Mary, who was the Principal, and Sister Mary Clarissa.

Further additions were completed in 1964, 1978, 1989, 1993. In the process it was renamed St. Gregory Catholic School to reflect the new parish name. By 2016 there were 12 classrooms and about 270 students.