Physics teacher (School Reunion)

The usual physics teacher at Deffry Vale High School was said to never have played the lottery when she won; she was given the ticket through the door at midnight. The Tenth Doctor replaced her, unlike the other new members of staff, whose predecessors, according to Mr Parsons, had the flu three months earlier. The Doctor simply responded, "The world is very strange." (TV: School Reunion)

Behind the scenes
The teacher's name was Victoria Mehru, according to the Deffry Vale School and Deffry Vale website.