The Deca was a group of ten young and rebellious Gallifreyan acolytes at the Prydonian Academy (AUDIO: The Rani Elite) who believed they could change the universe. (PROSE: The Quantum Archangel)
The Fifth Doctor had a dream about the Deca in which he was still in his first incarnation and back on Gallifrey. (PROSE: Divided Loyalties)
Overview
In the Academy, the members of the Deca questioned and debated the profound wisdoms and Ancient Covenants of Gallifrey. The Deca included the First Doctor, Koschei, Ushas, Mortimus, Drax, (PROSE: The Quantum Archangel) Vansell, (AUDIO: The Rani Elite) Rallon, Jelpax, Millennia, and Magnus.
Of these, Millennia and Rallon were made a part of the Celestial Toyroom, Jelpax became the Coordinator of the APC Net, (PROSE: Divided Loyalties) Vansell became head of the Celestial Intervention Agency, (AUDIO: The Sirens of Time) and the remaining six became renegade Time Lords.
The Fifth Doctor dreamed that the group represented the top rank of students in their class, (PROSE: Divided Loyalties) although the First Doctor was said to have not excelled academically. (TV: The Ribos Operation) By one account, the Doctor's low grades were a deliberate ploy to not to draw undue attention to himself, so he could eventually leave Gallifrey. (PROSE: Tears of the Oracle)