An Adelaide teacher allegedly dressed up as slain US conservative activist Charlie Kirk for a school Halloween event, in an incident described by the school's principal as "completely unacceptable".

In a statement issued by South Australia's Education Department, the principal of Urrbrae Agricultural High said the incident occurred at an event last Friday and that the teacher had subsequently been "directed away from the school".

The principal, Todd George, also said he had apologised to parents about the teacher's costume.

"On Friday a teacher made the completely unacceptable decision to dress as Charlie Kirk as part of a Halloween event with staff at the school," Mr George said.

"He remains away while an investigation is undertaken."

Charlie Kirk was shot and killed at Utah Valley U

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