In June 1943, Lorenzo converted and on 9 November 1943 entered the Cestello in Oltrarno seminary. The seminar years were quite hard, since very soon he began to collide with the Church views of the time: he could not understand the reasons of behaviours that he believed were far away from the Gospel. In December 1954, due to disagreements with the Florentine Curia, he was sent to Barbiana, a remote mountain hamlet in the municipality of Vicchio, in Tuscany, where he began a first attempt of full-time schooling, explicitly addressed to the working classes, where, among other things, he experimented a method of collective writing.
His school was housed in a couple of rooms in the rectory, adjacent to the small church of Barbiana, a few houses around the church and many others scattered on the slopes of Mount Giovi: in good weather he made outdoor lessons. The Barbiana school was a real collective where they worked together, and the main rule was that anyone who knew more helped and supported those who knew less, 365 days a year.
A fundamental achievement of the Barbiana school was "Lettera a una professoressa " (May 1967, translated as "Letter to a Teacher" by Nora Rossi and Tom Cole), where the kids of the school (along with Don Milani) denounced the school system and the teaching method which favored the education of wealthier classes. Written in the years of don Milani's disease, and published after his death, the book became a cult text for the 1968 student movement. Don Milani adopted the motto "I care", which will later be taken up by a number of religious and political organizations.