The headmaster or principal was the person in charge of a school.
H. Parson was the headmaster of Coal Hill School in November 1963. At this time, the Imperial Daleks had a implanted a device behind his ear to control him. When Mike Smith caught and was trying to question Parson, the Daleks used the device to kill him so he would not reveal any information. (TV: Remembrance of the Daleks)
DI Canyon was the headmaster of the Darkstar Academy in the 28th century. The Eleventh Doctor used his psychic paper to convince Canyon that the Doctor was actually a school inspector. (AUDIO: Darkstar Academy)