During Slann's reign as Lord President, survivors of the previous purge attempted to recruit the Doctor to their cause. Although the Doctor did not join them, the conversation was overheard and reported to Slann. Since the Doctor was innocent but highly regarded, he escaped the ensuing reprisals, but his memory was altered to remove his knowledge of the students' plot. Dismayed at the bloody purges that followed, the Doctor decided to leave Gallifrey — unknowingly taking the Lady Larn, a descendant of Rassilon who would have been used by the student rebels as a figurehead, with him.
Shortly after the Doctor's departure from Gallifrey, Lord President Slann was assassinated by the Master, who had been behind the earlier coup attempts. (PROSE: Birth of a Renegade)
The Presidents are categorised by in-universe chronological order. Loose chronological order applies for the lists, but when in doubt (as with the temporal uncertainty of the Time Wars), list them in the order in which they were first depicted as Presidents in the real world.