A Hong Kong primary school teacher has been de-registered after using pro-independence materials in class, reportedly to teach students about the concepts of freedom of speech and independence.

The education bureau accused the teacher of a premeditated act in violation of Hong Kong’s mini constitution, the Basic Law, by having “spread a message about Hong Kong independence”.

“In order to protect students’ interest and safeguard teachers’ professionalism and public trust in the teaching profession, the territory’s education bureau decided to cancel the teacher’s registration,” it said in a statement reported by the South China Morning Post.

Local media reports said the teacher had shown the class a video featuring a pro-independence activist, and had then asked the students questions including “what is freedom of speech”, and “according to the video, what is the reason for advocating Hong Kong independence?”.

The bureau said several teachers were warned over the incident, and that it would work to find other “black sheep” teachers accused of professional misconduct. Between July 2019, when the mass protests began, and August this year, the bureau received 247 complaints about teachers’ purported involvement with the protests. Of the 204 investigations concluded, 33 have resulted in reprimands or warning letters to teachers, and the bureau has not ruled out cancelling registrations for those found to have committed serious misconduct.

Advocating for independence in Hong Kong – the theme of mass protests which wracked the city through much of 2019 – is now illegal under the national security law imposed by Beijing more than three months ago. The broadly worded and ill-defined law has drawn international condemnation, resulted in dozens of arrests, and caused a chilling effect across schools and academia.

Hong Kong’s chief executive, Carrie Lam, who has driven a crackdown on dissidents and opposition in the region, said the case was a “very serious matter”.

“But if there are a very tiny fraction of teachers who are using their teaching responsibilities to convey wrong messages, to promote misunderstanding about the nation, to smear the country and the Hong Kong SAR [special administrative region] government without basis, then that becomes a very serious matter.”