Secondary school students pushing for Hong Kong's breakaway from China have threatened protests after staff stopped them from handing out political pamphlets, as the government puts pressure on independence activists.

Since the beginning of term last week, teenage students have been distributing leaflets outside their schools advocating Hong Kong's independence from China as concerns grow Beijing is tightening its grip on the semi-autonomous city.

The city was returned from Britain to China in 1997 under a "one country, two systems" deal that guaranteed its freedoms for 50 years, but there are fears those liberties are disappearing.

The fledgling independence movement has gained momentum in the face of staunch opposition from authorities in Beijing and Hong Kong, who have branded it illegal and unconstitutional.

Several young campaigners advocating independence or self-determination became lawmakers for the first time in weekend elections, despite a government ban on the movement's leading activists from running in the vote.

Secondary school students say they should have the right to distribute pro-independence pamphlets outside their schools, but have been stopped from doing so by staff.

"School staff...banned students from taking media interviews and even led away those who were distributing pamphlets," said a statement from an alliance of groups from 21 schools.

"If any more schools try to suppress members of the groups, we will definitely respond through escalated action," the statement said, without elaborating.

The South China Morning Post said student leaders had claimed they were planning to "lay siege" to schools.

The young activists say they want Hong Kong to have the option to split from China in 2047, when the handover deal expires.

"When 2047 comes, those who are in secondary school now will be middle aged," 15-year-old Tony Chung, a convener of new school activist group Studentlocalism, told AFP.

"We have to now discuss with students what is the way out for Hong Kong, and we think that way out is independence."

The "localist" movement of activists seeking more autonomy from Beijing first grew out of the failure of mass pro-democracy rallies in 2014 to win political reforms from Beijing.

That movement has now spawned campaigners calling for an outright split.

The government has warned teachers not to promote independence in schools, sparking criticism they are quashing academic discussion and fuelling self-censorship.

City leader Leung Chun-ying, seen by critics as a Beijing stooge, has likened promoting independence in schools to swearing.

"It is not a matter of freedom of speech to promote independence in schools...school rules are more strict than law," he said last month.

Secondary school students have in the past led major citywide campaigns.

Activist Joshua Wong, who became the face of the 2014 "Umbrella Movement" democracy rallies, first rose to prominence as a 15-year-old when he led student protests against a plan to introduce mandatory Chinese patriotism lessons at schools.

The government was forced to scrap the plan after 120,000 demonstrators rallied outside the government headquarters in September 2012.