As many as 2,000 teachers are at risk of losing their jobs when the new school term begins in September.
That's due to falling demand for places in secondary schools, according to a survey of government-run and subsidised schools by the Professional Teachers' Union.
The 47 schools that participated in the survey for more than 400 institutions said they would cull about 5.5 teaching positions on average from September.
The news comes a day after parents in the northern New Territories complained that they had failed to secure places for their children in primary schools because of competition from mainlanders.
On its part, the union has released an open letter to chief executive-elect Leung Chun-ying, urging him to halt the current administration's policy of closing schools that fail to meet a minimum enrolment number.
According to Education Bureau regulations, secondary schools must enrol 61 pupils a year to avert closure.
The warning of looming job losses among secondary school teachers maintains a trend seen in primary schools in recent years, as Hong Kong's low birth rate leads to lower enrolments.
But the trend, which has led to the closure of dozens of primary schools, has reversed this year, as mainlanders who gave birth in Hong Kong brought their children back to start their education - putting the number of limited school places under further pressure.
Speaking at a meeting of hundreds of teachers yesterday, union president Fung Wai-wah warned the government against closing secondary schools that fail to fill their rosters, as they will be needed when enrolment increases in the future.
The union also called on the chief executive-elect to implement small-class teaching as a way to stop the cull of teachers and improve the quality of education.