The film should not be shown, so as "to not impact on the delivery of planned lessons," wrote the NSW Education Minister, Adrian Piccoli in a memo to the schools.
Piccoli explained to a local radio station that he had insisted the screening at Burwood Girl's High School must happen after the school day was over.
"During school hours we expect them to be doing Maths and English and curriculum matters," Piccoli told 2GB Radio.
"This movie is not part of the curriculum and that’s why I’ve made that direction."
The film had been due to be shown to 1,200 students at the school in Sydney on Friday morning, as part of the country's Wear it Purple Day, a campaign to raise awareness about sexuality and gender diversity. The film's screening had been organized in up to 50 schools across Australia.
Local Presbyterian Minister Mark Powell said "many parents" were upset about lessons being canceled, in order for children to watch the film.
"This is trying to change children’s minds by promoting a gay lifestyle," Rev Powell said.
"Schools are supposed to be neutral and cannot propagate a political view."
On Wednesday the Premier of NSW said he supported his Education Minister's decision:
"I think tolerance is a good thing. But I think there should be some parameters around it," said Mike Baird.
"Should it be in class time? No, I don’t think so. Should it be optional? Yes, I do think so."