From the canonization documents

Joseph Calasanctius was the youngest of five children born to Maria Gaston and Pedro Calasanz in the family castle in Aragon, Spain. When he studied the humanities, his piety and virtue earned Joseph the disrespect of his classmates. Though his father hoped he would be a soldier, Joseph instead took up law and theology. Finally in 1583 he was ordained a priest. After various experiences which earned him many admirers, Calasanz was appointed vicar general and was extremely effective in reviving religion and reforming the clergy. Following a vision calling him to Rome, he left his fortune to family and charity and left Spain.

In Rome, Calasanz came under the patronage of Cardinal Colonna. During the plague of 1595, he distinguished himself by his courageous service to the sick and dying and entered into a "holy rivalry" with his friend St. Camillus of Lellis over who should give himself most freely in this charitable cause. For most of his first five years in Rome, however, Calasanz was concerned mostly with the instruction of poor children. He became aware of the appalling lack of education among the children of the poor and opened a free school for the religious and secular education of the poor. He was convinced that this was the basic and singular way in which a real reformation of the Church and of society could be attained. By 1602 the school had more than 700 students and was moved to a large house next to the church of Sant'Andrea della Valle. Calasanz broke his leg while hanging a bell, and was crippled and in pain the rest of his life.

The school's success and a grant from the Pope aroused the envy of the local parish-schoolmasters, who began to complain and criticize it. But a surprise inspection ordered by the Pope returned such positive reports that Pope Clement took the school under his protection, which was continued by Paul V. Other schools were soon opened and in 1621 the teachers were recognized as a religious order, the Clerks Regular of the Religious Schools, of which Calasanz became superior general. The congregation prospered and spread through Italy and into Europe, arousing further envy against Calasanz. In 1639 Fr. Sozzi, a member of the Order slandered Calasanz and even had him arrested and carried through the streets as a felon. Intervention by Cardinal Cesarini saved the 82 year old from prison, but Sozzi managed to take control of the Order himself and to have Joseph suspended from the Generalate. Calasanz was subjected to humiliating and insulting treatment during Sozzi's reign and that of his successor. Calasanz bore this treatment with patience and meekness, urging the order to obey his persecutors as the authority and even protected one from an angry mob of young priests who were enraged by his behavior. The Vatican, meanwhile, investigated the matter and in 1645, at age 88, Calasanz was reinstated as general of the Order. This victory was short-lived, however. Calasanz's enemies convinced Pope Innocent X to turn the control of the Order over to local bishops, thus dissolving the Order. Calasanz was reported to have said, upon hearing the news, "The Lord gave and the Lord has taken away. Blessed be the name of the Lord." Calasanz died a few days before his 92nd birthday. A few years after his death, the Order was reconstituted and restored.

Calasanz was canonized in 1767. For his heroic patience in adversity, he has been called another Job, "a perpetual miracle of fortitude." Joseph Calasanz was declared patron of all Christian schools by Pope Pius XII.