The mother of a girl who refused to do a school assignment to plan a fake terrorist attack on "innocent civilians", says she's proud her daughter stood up for her beliefs.

A society and environment teacher at the Kalgoorlie-Boulder Community High School asked Year 10 students to pretend they were a terrorist planning a chemical or biological attack on "an unsuspecting Australian community".

"Your goal is to kill the MOST innocent civilians in order to get your message across," the assignment read.

The students had to explain their choice of victims and decide the best time and place to strike.

But the assignment was withdrawn and the teacher counselled following a complaint to the school after 15-year-old student Sarah Gilbert refused to do it, saying she was horrified and disgusted.