Lassar

Brother Lassar was a Krillitane who, under the name Lucas Finch, took over Deffry Vale High School in order to crack the Skasis Paradigm, which would make both he and his brothers "gods". He was a very sinister alien and liked eating children for lunch. Eventually, his plan was foiled and he was believed to have perished along with the other Krillitanes when K-9 blew up a can of Krillitane oil, destroying both the Krillitanes and the school. (DW: School Reunion)

However, Lassar survived the blast and later, now permanently in human form, served as prosecutor when the Doctor was put on trial by the Shadow Proclamation. (IDW: Fugitive)

Behind the Scenes
Finch's first name was not given onscreen, only on the Deffry Vale School website. According to an on-line interview with Head, Finch's original name in the script was "Hector", but this had to be changed when a check found a real headmaster named "Hector Finch". However, Finch's name is given as Hector in Doctor Who: The Encyclopedia.

When the cast was announced, the character's name being given as 'The headmaster' led to speculation that Head was playing a new regeneration of The Master.