Since the 19th century, Catholic secondary schools in Brittany have demonstrated exceptional vitality, the reason for which can be highlighted through a new approach grounded in the serialisation of memos concerning the history of those institutions which have become lycées, high schools preparing for university entrance examinations. Until the 60s, their dynamism is based mainly on the singular position occupied by the clergy in society, both quantitatively and qualitatively. The fall in vocations did not modify its attractivity, whose second explanation is in its reputation for excellence, inherited from the clerical experience. But the success of Catholic secondary schools is also due to their institutional nimbleness, which allows them to adapt quickly to expectations, whether in rich city centres or in rural settings. To sum up the situation: private schools were attractive because they were catholic, and catholic schools are attractive because they have the freedom to adapt.