Protest in Hong Kong

St Francis Xavier's School said that besides failing to attend the assembly on Wednesday, the pupils had "committed disrespectful acts", without providing further details.

Media reports said some of the pupils were having breakfast in the covered playground when the national anthem began to play.

Speaking on Commercial Radio on Tuesday, education sector lawmaker Chu Kwok-keung said class suspension was a "quite serious" penalty.

While he refused to comment directly on the incident, he said that schools should weigh how a punishment can help pupils recognise their mistakes.

"The best way is to use the appropriate penalties to teach them within the school, as class suspensions may affect their learning and they may not be able to catch up on progress," Chu said.

The Education Bureau said it was seeking a report on the incident from the school. It said there are clear rules governing etiquette during the national anthem and flag-raising ceremony.

Unidentified men came out of the consulate and forced a man inside the compound, before he escaped with the help of police and other demonstrators.

The protester told the BBC: "They dragged me inside, they beat me up."

A consulate spokesperson said protesters had displayed an insulting portrait of China's president.

The Foreign Office said it was urgently seeking clarity on the incident. Greater Manchester Police has launched an investigation.

Speaking after the incident, the protester, called Bob, told BBC Chinese that "mainlanders" - people from mainland China, as opposed to Hong Kong - came out of the consulate and destroyed their posters.

"As we tried to stop them, they dragged me inside, they beat me up," he said, adding that he was then pulled out by the UK police.

"It's ridiculous. They [the attackers] shouldn't have done that. We are supposed to have freedom to say whatever we want here [in the UK]."

After the incident, the crowd remained angry. Protesters shouted at the men from the consulate and the British police, arguing they could have done more.

Consulate staff had previously asked the protesters to move to the opposite side of the street.

There were two police officers at the protest, but several more appeared within minutes of the altercation beginning.

They gathered at the gates of the compound trying to break up the fighting and move protesters back.

One police officer entered the consulate grounds and pulled the man who had been dragged inside back out.

At least eight men - some of whom were wearing helmets and protective vests - then returned to the consulate building.

The consulate is on UK soil, but cannot be entered without consent.

Reacting on Twitter, former Conservative leader Iain Duncan Smith said the UK government should demand a full apology from the Chinese ambassador, and that those involved should be sent back to China.

Protests outside embassies and consulates in Britain often involve a few scuffles. Police are often on hand to keep the peace.

But rarely do consular staff emerge on to the street to pull down banners and posters. And even more rarely are protesters dragged through the gates and apparently beaten up.

So it is not surprising there are growing calls for the Foreign Secretary, James Cleverly, to summon China's ambassador in Britain for an explanation. Labour's foreign affairs spokesman, David Lammy, said the alleged assault was unacceptable. "The quashing of peaceful protest will never be tolerated on our streets," he said.

The police and the Home Office will investigate first and decide what action to take - if any - from a criminal justice point of view. As for the Foreign Office, it will have to decide whether to make any diplomatic response.

Britain is, of course, a signatory to the Vienna Convention, which gives some diplomatic immunity to consular staff and their properties. But diplomats and their employees are still covered by UK law and can potentially be declared persona non grata by the British government.

The demonstrators were protesting as the ruling Communist Party congress began in Beijing.

President Xi Jinping, who is set to secure a third term in power, said he had turned the situation in Hong Kong from "chaos to governance", referring to China's suppression of pro-democracy protests there.

A spokesperson for the consulate said the protesters had "hung an insulting portrait of the Chinese president at the main entrance".

"This would be intolerable and unacceptable for any diplomatic and consular missions of any country. Therefore, we condemn this deplorable act with strong indignation and firm opposition," the spokesperson added.

A spokesperson said Greater Manchester Police was aware of the incident.

"Officers were present and responded immediately to defuse the situation," they said.

"Enquiries are ongoing at this time to understand the full circumstances."