For his last class in Hong Kong in July, liberal studies teacher Fong showed his students calligraphy by the territory's late democratic activist Szeto Wah: "Choose the right path and stick to it." He emigrated to Britain days later.

Fong is one of many teachers that left Hong Kong before the school year began in September, some saying they felt disillusioned and threatened by the authoritarian turn the city has taken since Beijing imposed a stringent national security law in June 2020.

"The day I resigned, I told my school...