The surname of South Park Elementary teacher Mr. Garrison is Garrison. His full name is Herbert Garrison, although the character has used other names and identities during the series.
Garrison is one of South Park's most changeable characters. He begins as the boys' fourth-grade teacher, later undergoes a gender transition and uses the name Janet Garrison, and eventually returns to being known as Mr. Garrison. He also becomes President of the United States for several seasons.
The character first appears in “Cartman Gets an Anal Probe,” the show's 1997 premiere. Trey Parker voices Garrison and based elements of the character on teachers from his own life, including a kindergarten teacher who used a hand puppet named Mr. Hat.
The wrong options can seem familiar because Mr. Mackey is another South Park Elementary staff member, while Mr. Slave is a character connected to Garrison's personal life. Adler is also a teacher, but not Garrison's surname.