From Lexicanum

The Schola Progenium are schools run by Missionaries of the Ecclesiarchy. These schools teach and train orphans of Imperial officials who have given their lives in the service of the Emperor. Orphans are educated by the Missionaries and receive a strong Imperial cult upbringing, and soon learn to regard the Emperor as their spiritual father. Graduates of the Schola are known as Progena, and because of the loyalty their training has instilled in them, graduates are sent for further training and service within another Imperial organization, often becoming Storm Troopers and Commissars in the Imperial Guard, Imperial assassins, and Inquisitors. Many girls join one of the Orders of the Adepta Sororitas.

Sources

• White Dwarf 115, 1989, Commissar Training Squads, reprinted in Warhammer 40,000: Compilation, 1991
• Codex Imperialis, by Rick Priestley