Wikipedia
A minor seminary is a secondary boarding school created for the specific purpose of enrolling teenage boys who have expressed interest in becoming priests. They are generally Roman Catholic institutions, and designed to prepare boys both academically and spiritually for vocations to the priesthood and religious life. They emerged in cultures and societies where literacy was not universal, and the minor seminary was seen as a means to prepare younger boys in literacy for later entry into the major seminary.
The minor seminary is no longer very familiar in the English-speaking world, as it once was in Europe. The 1917 Code of Canon Law described the purpose of minor seminaries as: "to take care especially to protect from the contagion of the world, to train in piety, to imbue with the rudiments of literary studies, and to foster in them the seed of a divine vocation". Suitable boys were encouraged to graduate to a major seminary, where they would continue their tertiary studies for the priesthood.
Today, college seminaries, where philosophy is studied, are called minor seminaries even though they are for those who have completed high school.