There is no evidence that three girls, thought to be heading to Syria to join Islamic State, were radicalised at school, their principal has said.
Mark Keary said pupils cannot access Twitter or Facebook on Bethnal Green Academy computers.
"Police have advised us there is no evidence radicalisation took place at the academy," he said.
Shamima Begum, Amira Abase, both 15, and Kadiza Sultana, 16, flew from London to Turkey on Tuesday.
UK police officers have gone to Turkey but their role has not been confirmed.
The girls were all studying for their GCSEs at the east London school, which reopened on Monday after half term.
The head teacher said the school was "shocked and saddened" by the girls' disappearance.
"This situation follows the earlier disappearance of a student in December of last year," he said.
"The police spoke to the student's friends at that time and, further to this, they indicated there was no evidence the girls were at risk of being radicalised or absconding."
Mr Keary said it was business as usual for the 1,200 pupils and staff, although "a full programme of briefing sessions" with police and counter-radicalisation groups was available.
"The priority for all of us is the safe return of the girls," he added.
The girls boarded a Turkish Airlines flight from Gatwick, and would have needed a visa for Turkey and a passport.
It has emerged that Shamima used the passport of her 17-year-old sister Aklima to travel.
Security services have been criticised after it emerged that, before leaving the UK, Shamima sent a Twitter message to Aqsa Mahmood, who left Glasgow for Syria in 2013 to marry an Islamic State fighter.
According to a lawyer for Ms Mahmood's family, her Twitter account has been "monitored" by police since she left Britain.
He said authorities should have seen Shamima's message and taken action before she and her two friends followed.
Their families have made appeals for them to come home.
Another of Shamima's sisters, Renu Begum, said she hoped her sister had gone to Syria to bring back the girl who had gone there from Bethnal Green Academy in December.
Ms Begum said Shamima and her friends were "young" and "vulnerable" and if anyone had tried to persuade them to go to Syria it was a "cruel and evil" thing to do.
'We miss you'
Amira's father, Abase Hussen, said: "The message we have for Amira is to get back home.
"We miss you. We cannot stop crying. Please think twice. Don't go to Syria."
In an appeal to Kadiza, her sister, Halima Khanom, said: "Find the courage in your heart to contact us and let us know that you are safe and you are OK.
"That is all we ask of you."