HONG KONG — High school teacher Dom Chan had an odd request from two superiors while developing next year's Chinese history syllabus: remove passages from the philosopher Su Xun, known for 11th-century essays on wars and military reforms.

"They told me, 'You need to scan the textbook carefully' " because Su's writings could "incite violence in students or make them think revolution is good," Chan said.

Despite his misgivings about erasing a literary luminary of the Song dynasty, the 26-year-old complied.

As China's Communist Party dismantles Hong Kong's freedoms, teachers are facing pressure to toe Beijing's line. Schools are emerging as ideological battlegrounds as officials seek to transform freethinking students into patriots loyal to the motherland through punishment, coercion, surveillance and propaganda-style education.

"I feel like we have suddenly been put on the front line," said Chan, whose Chinese history syllabus officials have made compulsory for the first time. "The government seems to have found that education is easier to blame for the current situation in Hong Kong, and easier to fix."

A culture of self-censorship and government control that was already growing in schools intensified recently as Beijing introduced a security law aimed at eliminating dissent, according to nearly a dozen teachers and students who spoke to The Washington Post. The law, published June 30, compels Hong Kong's government to "promote national security education" and pinpoints campuses for "supervision and regulation."

This week, education authorities told schools to review their library collections and remove books that could violate the law. Titles including those by democracy activist Joshua Wong have already disappeared from public libraries.

The security law is so feared in Hong Kong, which was rocked by anti-government protests last year, that there has been no discussion of it in classrooms or among teachers and principals, teachers say. Instead, classroom debates on political issues — common until recently — have been replaced by silence on the most consequential changes since the city's 1997 handover to China.

Most teachers spoke on the condition of anonymity, fearing retribution over their political views.

"It is getting to the point where we are wondering if we should quit," said a 50-year-old who teaches history and liberal studies, the latter of which pro-Beijing lawmakers have blamed for radicalizing students. "What is the point of teaching if all they want us to do is manipulate, brainwash and control our kids?"

'We are fighting a war'

Leading Beijing's charge is Hong Kong's Education Bureau, which has issued new directives in recent weeks.

When a law criminalizing mockery of China's national anthem took effect in June, Education Secretary Kevin Yeung said Hong Kong teachers should be ready to call the police on students who disrespect the song. He told a local radio station that "Glory to Hong Kong," the democracy movement's unofficial anthem, should be banned in schools because it was "propaganda." Officials required schools to display the Chinese and Hong Kong flags during celebrations and play China's anthem, "March of the Volunteers."

In May, the bureau ordered the local examinations body to remove a question on Sino-Japanese history, saying it was biased and would hurt the feelings of Chinese people. And for the next school year, teachers will have to undergo new training on professional "roles, values and conduct."

"What the government is trying to say is that there are wrong teachers, wrong materials, wrong subjects and recently there are wrong examinations, and these are the reasons why the young people have been led to a wrong path," said Ip Kin-yuen, who represents the education sector in Hong Kong's legislature. "Obviously, we do not agree."

The Education Bureau did not respond to a request for comment and declined to make the secretary available for an interview.

Teachers say they fear the bureau because it is responsible for registering teachers, including those in private and international schools. Several said the bureau has ordered schools to take action against them even when their principal or school board has found no wrongdoing. Some, including a liberal studies teacher and artist who posts pro-democracy sketches on his personal Instagram page, have been terminated.

"We teachers can't do anything about that," said Raymond Yeung, a 30-year-old liberal studies teacher who was partially blinded in one eye when he was hit by a tear-gas canister at a protest last year. He said the elite girls school where he taught had indicated it would not extend his contract after it canceled his subject as a compulsory course for some students.

"It has come to the point in our movement that we are fighting a war, and they will use whatever method they can to suppress us," Yeung said.

History vanishes

Historical moments and topics of discussion are disappearing from official textbooks, teachers say. In liberal studies, textbooks formerly prompted discussion on whether civil disobedience is within the rule of law, citing the examples of the Rev. Martin Luther King Jr. and Mahatma Gandhi. This was removed, according to teachers, after Hong Kongers protested in 2014 for universal suffrage.

Some schools have warned teachers against discussing China's 1989 massacre of students in Tiananmen Square. The Education Bureau's teaching materials discuss the Cultural Revolution and Mao Zedong's purges of intellectuals and other anti-communist elements in one paragraph.

Topics that could veer into criticism of Hong Kong leaders and institutions have also been erased. Some teachers said they are skipping topics on the role of police. At Yeung's school, discussion of Lee Tung Street — an urban-renewal project championed by Hong Kong Chief Executive Carrie Lam that turned a street of wedding-card vendors into a sanitized strip of high-end shops — was removed.

"The devil's claw is gradually stretching into education," said one elementary school teacher. "They are trying to control the independent thinking of the students, to influence the way they judge right from wrong."

Prank calls and death threats

Teachers also described instances of doxing, threats and harassment. More than 40 teachers have received official warnings from the Education Bureau for social media posts perceived to be critical of the Hong Kong police and government since late last year, according to Ip.

Ip said teachers are self-censoring, unsure "who is complaining and who is digging out such things, in some cases, even from their private social media pages."

Yeung, the 30-year-old teacher, said he received death threats, prank calls and pictures of dead bodies after reports emerged that he had been injured at a protest. Recently, men have started ringing his doorbell and running away. He has installed security cameras.

"They have been very effective," Yeung added. "In one year, they've been able to scare so many teachers, and so many of us have been affected, but most are too afraid to speak up."

A 24-year-old high school mathematics teacher, who was arrested and charged last July for nonviolent offenses related to the protests, said the Education Bureau sent letters to her principal asking whether the school had considered suspending her.

The principal allowed her to continue. But after the teacher's personal information was posted on a doxing website, she received late-night calls from strangers who accused her of prostitution and of having sex with protesters. One person mailed a letter to her home containing nude photos, falsely claiming that they were of her students and that she had encouraged the behavior.

She, too, has installed security cameras.

Some educators have found ways to push back. One elementary school teacher, unsatisfied with class materials that used Mandarin and the simplified Chinese characters used in mainland China, created videos in Cantonese with subtitles in traditional characters used in Hong Kong. Others are writing textbooks or creating support groups with pro-democracy parents so they can teach children privately about events such as Tiananmen.

"You can try to stop students from singing songs and waving flags," said the 50-year-old history and liberal studies teacher, "but at the end of the day, you can't control their minds."

Tiffany Liang contributed to this report.