Nazir Afzal, who stepped down as North West England chief prosecutor last month, said that at least two headteachers in London told him of their concerns.
"One head said he was being approached almost on a daily basis by parents who fear their kids are about to go to Syria. Both said they were scared of the Easter break and would be relieved if all their pupils came back after the holidays," Afzal was quoted as saying by The Times.
According to the US National Counterterrorism Center, the militant group has attracted at least 20,000 foreign fighters.
In February, a militant known as "Jihadi John", who appeared in several ISIL beheading videos, was identified as British citizen Mohammed Emwazi.
The Islamic State is a radical Sunni militant group in control of large areas in Syria and Iraq. It is notorious for its many human-rights atrocities, including on-camera executions of foreign hostages posted online.
A US-led Coalition of some 60 countries has been carrying airstrikes against the ISIL in Iraq and Syria since August 2014.
