A teacher was a learned individual employed to teach others, typically in an institution, such as a school.

Two of the First Doctor's first companions were teachers. In November 1963, Ian Chesterton and Barbara Wright, who taught Susan Foreman, followed her to a junkyard as they were worried for her safety. They quickly became caught up in the Doctor's travels. (TV: An Unearthly Child)

The Second Doctor, Jamie McCrimmon, and Zoe Heriot, when visiting Dulkis, met Balan, a teacher and researcher of the residual radioactivity from a blast that happened 172 year prior. (TV: The Dominators)

The Third Doctor befriended Bernard Helmann, a philosophy teacher at Oxford, at the 1960 Campaign for Nuclear Disarmament. (PROSE: Come Friendly Bombs...)

The Seventh Doctor and Ace met Miríl, a teacher on Kirith, and uncovered the truth about the planet's history with him. (PROSE: Timewyrm: Apocalypse)

In 2007, the Tenth Doctor posed as and met many teachers while investigating Deffry Vale High School with Rose Tyler, Mickey Smith, and Sarah Jane Smith. (TV: School Reunion)

In 1913, the Tenth Doctor became the human teacher John Smith at the Farringham School for Boys for a short time when travelling with Martha Jones. (TV: Human Nature/Family of Blood)

Clara Oswald, companion of the Eleventh Doctor, became a teacher at Coal Hill School. (TV: The Day of the Doctor) At the school she dated Danny Pink, another teacher. (TV: Into the Dalek, Listen, The Caretaker)

The Voord Nebrin's father was a teacher. (AUDIO: Domain of the Voord)

The Twelfth Doctor became a teacher at St Luke's University. (TV: The Pilot)

According to one account, Rose Tyler became a teacher on Pete's World. (PROSE: The Turning of the Tide)