In a further sign of China’s increasingly thin skin when it comes to its freest city, teachers in Hong Kong were warned this week that calling for the semiautonomous territory’s independence in schools could see their teaching credentials revoked.

The city’s Education Bureau said Sunday that “no pro-independence advocacy or activities should appear in schools … and any organization which serves to promote independence must be banned,” according to the South China Morning Post.

The bureau was responding to a statement issued by Hong Kong’s Academy of School Managers denouncing the emergence of separatist-advocacy groups at several secondary schools, calling their actions a “violation of educational ethics.”

“For teachers involved in unlawful activity or misconduct, the bureau will follow its existing mechanism in reviewing the qualifications as the registered instructor,” a spokesperson for the governing body added.

Read More: Hong Kong Makes History With First Pro-Independence Rally

The move signals the establishment’s alarm at Hong Kong’s growing independence movement, which has gone from a nascent, fringe campaign to a call supported by — according to at least one poll — 1 in 6 citizens. Thousands of those supporters made history earlier this month at the city’s first formal pro-independence rally, boldly demanding their sovereignty.

The announcement came on the same day that Edward Leung, one of Hong Kong’s most prominent separatist voices, was involved in a violent altercation with another man at a local subway station. A viral video of the incident shows them scuffling and cursing before being pried apart by staff, but the man — reportedly a journalist for a pro-Beijing newspaper — has filed police charges saying Leung assaulted him. The 25-year-old activist declined to comment on the incident but told the Post he has been “followed for more than a month” and as such has “no privacy.”

Leung has also been the primary target of a crackdown by Hong Kong authorities on pro-independence leaders, with the Electoral Affairs Commission recently barring him and several others from contesting in elections to the legislature slated for next month.

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Write to Rishi Iyengar at rishi.iyengar@timeasia.com.