The Indian Missionary Society is an indigenous Religious Order of the Catholic Church founded by Rev. Fr. Gaspar A. Pinto in 1941. Rev. Fr. Gaspar A. Pinto was born on 29th December 1905 in Tudam village in the Diocese of Mangalore. The IMS educational objective is to bring up men and women who have a deep faith in Almighty God, imbued with this faith to work for justice and equality. Keeping this in mind IMS fathers of Ranchi region have established few schools in which young men and women belonging to every social status, community and linguistic groups are educated through the medium of English and other languages.

St. Mary's School, Sisai is an English Medium Catholic School for the boys and girls owned and run by the Fathers of Ranchi Region of the Indian Missionary Society, which also constitutes the governing body of the school. As a catholic school, St. Mary's school has its inspiration and guidance from the life and teaching of Jesus Christ. Special attention is paid to faith formation of the catholic students through religion, moral classes, prayer and charismatic retreat

It is a minority Christian School, open to the children of allcommunities with a special responsibility for the education of Christians, tribals and the economically and socially weaker section of the society.

The school receives no government or private aid, and fees are its only income. Hence fees have to be adjusted from time to time to keep up with rising costs and salaries.