|Southwell Minster in Nottingham, home of a noisy school|
This did not start in 1215, actually: the Third Lateran Council of 1179 (called by Pope Alexander III) had already declared that it was the duty of the Church to provide free education "in order that the poor, who cannot be assisted by their parents' means, may not be deprived of the opportunity of reading and proficiency."
One wonders how carefully churches complied with this. Because the school was integral to the church it was attached to, records are not as abundant as they might be if the school were a separate legal entity with its own building, property taxes, et cetera. We have to look for more anecdotal and incidental evidence.
Among Roger Bacon's unedited works is a reference about schools existing "in every city, castle and burg." John of Salisbury (c.1120-1180), English author and bishop, mentions going with other boys as a child to be taught by the parish priest. (Note that this is long before the Lateran Council decrees; it seems they may have simply affirmed and extended a long-held practice.)
Schools for young boys stayed attached to churches for a long time. A late-medieval anecdote of Southwell Minster in Nottinghamshire (believed to be the alma mater of Thomas Cranmer, Archbishop of Canterbury under Henry VIII) tells that a visiting clerk (priest) complained that the noise of the boys being schooled was so great that it disturbed the services taking place. And Shakespeare's Twelfth Night acknowledges these schools with the line "Like a pedant that/Keeps a school i' the Church." It would be a long time before schools for the young were deemed to need their own buildings.