The headmaster of a London school attended by three girls believed headed for Syria on Monday called for their return as police briefed staff and students on the threat of radicalisation.

Mark Keary, headmaster of east London's Bethnal Green Academy, said the visiting police officers were part of a "support team" from the government's Prevent programme called in following the girls' disappearance last week.

Around ten police stood on duty outside the secondary school, as pupils made their way back to school for the first time since police on Friday launched an appeal to find the missing teenagers.

Keary said police had indicated there was "no evidence that radicalisation of the missing students took place at the academy".

"It is clear that this is an international issue which is increasing in severity and it's affecting schools across the country and beyond," he said.

Kadiza Sultana, 16, and 15-year-olds Shamima Begum and Amira Abase, left their homes last week and flew to Istanbul, raising concerns they would travel on to Syria to join Islamic State jihadists.

Prime Minister David Cameron on Monday said he was "horrified by the way that British teenagers appear to have been radicalised and duped by this poisonous ideology of Islamist extremism while at home on the internet in their bedrooms.

"They appear to have been induced to join a terrorist group that carries out the most hideous violence and believes girls should be married at nine and women should not leave the home," he told parliament.

- Mosque appeal -

Around 500 British nationals are believed to have travelled to Iraq and Syria to join the IS group.

Sultana and Begum are British nationals, while Abase is a German citizen.

The three were all friends with another student at the school who left for Syria in December, one of some 550 Western women believed to have gone to Syria.

"Our thoughts are with the families of the missing girls as we await news and hope for their safe return," Keary said.

He added that he would seek to ensure that school life for the 1,200 students and staff at Bethnal Green Academy was "business as usual".

Police have made an unprecedented appeal for the girls to come home but the case has sparked a heated debate over whether police could have acted sooner and whether warning signs were missed.

Keary said the school had "an outstanding system of pastoral care" and restricted access for students to Facebook and Twitter on its computers.

Someone using a Twitter account in Begum's name last Sunday seems to have contacted Aqsa Mahmood, a woman from Glasgow, Scotland, who reportedly travelled to Syria last year to marry an IS group fighter.

"A core aspect of our ethos is to promote the British values of democracy, tolerance and respect, particularly respect for other cultures," Keary said.

On Friday there was a call by the nearby East London Mosque for worshippers to help track the girls.

"There was an appeal in the mosque, that if anyone has any information, they should come forward," said worshipper Suleman Shahbaz, who explained that the local Muslim community was being educated in the fight against radicalisation.

"I went to a talk last week and we were given guidance, so if we come across individuals that are maybe persuading the younger generation to do unlawful things, then we should desperately speak out," he said.

Britain has prosecuted people for preparing to join IS and the authorities have seized passports from would-be jihadists under powers that have been reinforced by a new security law passed this month.