Magnus (Divided Loyalties)

According to one account, in his original incarnation, Magnus, the Time Lord who would later become the War Chief attended the Prydonian Academy as part of the Deca. Other members included Theta Sigma, later the Doctor, and Koschei, later the Master, (PROSE: Divided Loyalties) in contrast to accounts suggesting the War Chief and the Master were incarnations of the same Time Lord. (PROSE: Doctor Who and the Doomsday Weapon, etc.)

Behind the scenes
In a deleted line from the AUDIO: Neverland script, the Eighth Doctor mentions both Magnus and a certain "Koschei" when reminiscing about the Deca.