In the meantime the Roman Catholic Separate schools have been keeping pace with the growth of Oshawa. Catholic parents of the early days as well as at the present time wanted their children to have a religious education in Separate schools.
Around the year 1855, Father Proulx, who was the first resident priest in Oshawa, engaged a Mr. Cullen to hold classes for Roman Catholic students in the Sons of Temperance hall. This continued until 1858. In that same year Father Proulx requested the Sisters of St. Joseph of Toronto to come to Oshawa in November. They formed a new mission here and permanently took charge of the school. Since then they have had the care and directing of the schools until 1920. The need for teachers became so great then, that they have since been assisted by lay teachers.
In 1858, Rev. Mother Frances McCarthy and Sister Aloysius Tuite held classes for their thirty pupils in the sacristy of the church for a few months. However they had to move to the body of the church on account of the increase in the number of pupils. The church and school was called St. Mary until 1894 when the present Church was built. Then the names of both were changed to St. Gregory the Great. Father M.J. Jefficott was the priest in charge at that time.
The Sisters were pleased to see a small schoolhouse built in the spring of 1859. It was a one-story brick structure, some reports say it was a frame building and was situated behind the old church.
This school house was replaced in the year 1867 by a two-story white brick school. It was on Simcoe Street north near the site of the present one and was quite close to the street. It had red facing and was small, measuring 30 ft. by 40 ft. It was complete with a tower and bell. The total cost was $1500. The May Brothers were the masons and J. and A. Thomas were the carpenters. The school accommodated fifty or sixty pupils.