Upvote:3
The earliest mention we have to Cathedral Schools is from the Second Council of Toledo in 527; they were set up in order to guarantee the formation of priests after the fall of the Empire caused the collapse of the Roman system of education. After all, priests must at least be able to read, and after the general emptying of the cities in the Early Middle Ages it was a rare skill to find in the general population.
Note that this does not mean that priests didn't receive formal education during the Imperial era; but we do not have information of schools specific for priest formation. But starting from the 6th Century, we have the formation of a string of schools which provide an education which has become the formal Seminary system for priestly formation (as well as the University system).
Upvote:7
First let me correct the assumption that the model you describe is normal for Christian pastors. It is in fact only a minority that follow this. The vast majority of Christian churches, including Catholic, Anglican, Lutheran and Presbyterians follow an entirely different model. In this model they:
Even among those churches where the model you describe is used, it is not exclusive. I have encountered Baptist, Mennonite and other churches where they will find a person who is gifted in a pastoral or leadership area and appoint them to that role. They may or may not then send them for other training (formal or informal) as part of their duties.
However to start answering your question, which is about when it became normal to give church leaders formal education, it has been normal for Church leaders to receive formal education for as long as there has been formal education. Early universities were started as clergy training institutions, and only later developed into general education institutions.